<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.mrcy.com/role/Acquisitions" xlink:href="mrcy-20180331.xsd#Acquisitions" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/AcquisitionsNarrativeDetails" xlink:href="mrcy-20180331.xsd#AcquisitionsNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/AcquisitionsNetPurchasePriceAndFairValuesOfAssetsAndLiabilitiesAcquiredDetails" xlink:href="mrcy-20180331.xsd#AcquisitionsNetPurchasePriceAndFairValuesOfAssetsAndLiabilitiesAcquiredDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/AcquisitionsProFormaDetails" xlink:href="mrcy-20180331.xsd#AcquisitionsProFormaDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/AcquisitionsTables" xlink:href="mrcy-20180331.xsd#AcquisitionsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/ChangesInCarryingAmountOfGoodwillDetail" xlink:href="mrcy-20180331.xsd#ChangesInCarryingAmountOfGoodwillDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/CommitmentsAndContingencies" xlink:href="mrcy-20180331.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/CommitmentsAndContingenciesAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#CommitmentsAndContingenciesAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/ConsolidatedBalanceSheets" xlink:href="mrcy-20180331.xsd#ConsolidatedBalanceSheets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:href="mrcy-20180331.xsd#ConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/ConsolidatedStatementsOfCashFlows" xlink:href="mrcy-20180331.xsd#ConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:href="mrcy-20180331.xsd#ConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/Debt" xlink:href="mrcy-20180331.xsd#Debt" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/DebtAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#DebtAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/DescriptionOfBusiness" xlink:href="mrcy-20180331.xsd#DescriptionOfBusiness" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/DescriptionOfBusinessDetails" xlink:href="mrcy-20180331.xsd#DescriptionOfBusinessDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/DocumentAndEntityInformation" xlink:href="mrcy-20180331.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/EmployeeBenefitPlanDetails" xlink:href="mrcy-20180331.xsd#EmployeeBenefitPlanDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/EmployeeBenefitPlanEmployeeBenefitPlan" xlink:href="mrcy-20180331.xsd#EmployeeBenefitPlanEmployeeBenefitPlan" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/FairValueOfFinancialInstruments" xlink:href="mrcy-20180331.xsd#FairValueOfFinancialInstruments" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/FairValueOfFinancialInstrumentsSummaryOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetail" xlink:href="mrcy-20180331.xsd#FairValueOfFinancialInstrumentsSummaryOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/FairValueOfFinancialInstrumentsTables" xlink:href="mrcy-20180331.xsd#FairValueOfFinancialInstrumentsTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/Goodwill" xlink:href="mrcy-20180331.xsd#Goodwill" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/GoodwillTables" xlink:href="mrcy-20180331.xsd#GoodwillTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/IncomeTaxes" xlink:href="mrcy-20180331.xsd#IncomeTaxes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/IncomeTaxesAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#IncomeTaxesAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/Inventory" xlink:href="mrcy-20180331.xsd#Inventory" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/InventoryAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#InventoryAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/InventoryDetail" xlink:href="mrcy-20180331.xsd#InventoryDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/InventoryTables" xlink:href="mrcy-20180331.xsd#InventoryTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomers" xlink:href="mrcy-20180331.xsd#OperatingSegmentGeographicInformationAndSignificantCustomers" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersCustomersComprisingTenPercentOrMoreRevenuesDetail" xlink:href="mrcy-20180331.xsd#OperatingSegmentGeographicInformationAndSignificantCustomersCustomersComprisingTenPercentOrMoreRevenuesDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersGeographicDistributionOfRevenuesAndLongLivedAssetsFromContinuingOperationsDetail" xlink:href="mrcy-20180331.xsd#OperatingSegmentGeographicInformationAndSignificantCustomersGeographicDistributionOfRevenuesAndLongLivedAssetsFromContinuingOperationsDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersProgramsComprisingTenPercentOrMoreOfCompanysRevenueDetail" xlink:href="mrcy-20180331.xsd#OperatingSegmentGeographicInformationAndSignificantCustomersProgramsComprisingTenPercentOrMoreOfCompanysRevenueDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersTables" xlink:href="mrcy-20180331.xsd#OperatingSegmentGeographicInformationAndSignificantCustomersTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/Restructuring" xlink:href="mrcy-20180331.xsd#Restructuring" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/RestructuringAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#RestructuringAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/RestructuringExpensesByReportableSegmentForRestructuringPlansDetail" xlink:href="mrcy-20180331.xsd#RestructuringExpensesByReportableSegmentForRestructuringPlansDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/RestructuringTables" xlink:href="mrcy-20180331.xsd#RestructuringTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensation" xlink:href="mrcy-20180331.xsd#StockBasedCompensation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensationAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#StockBasedCompensationAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensationStockBasedCompensationExpensesDetail" xlink:href="mrcy-20180331.xsd#StockBasedCompensationStockBasedCompensationExpensesDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensationSummaryOfNonvestedRestrictedStockDetail" xlink:href="mrcy-20180331.xsd#StockBasedCompensationSummaryOfNonvestedRestrictedStockDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensationSummaryOfStockOptionPlansDetail" xlink:href="mrcy-20180331.xsd#StockBasedCompensationSummaryOfStockOptionPlansDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/StockBasedCompensationTables" xlink:href="mrcy-20180331.xsd#StockBasedCompensationTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SubsequentEvents" xlink:href="mrcy-20180331.xsd#SubsequentEvents" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="mrcy-20180331.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" xlink:href="mrcy-20180331.xsd#SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesBasicAndDilutedWeightedAverageSharesOutstandingDetail" xlink:href="mrcy-20180331.xsd#SummaryOfSignificantAccountingPoliciesBasicAndDilutedWeightedAverageSharesOutstandingDetail" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="mrcy-20180331.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:href="mrcy-20180331.xsd#SummaryOfSignificantAccountingPoliciesTables" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/Acquisitions" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/AcquisitionsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_BF3D1BFA89998DC5A7511933ED378535" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_BF3D1BFA89998DC5A7511933ED378535" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_6EBD60A9949E615AEC2B1933ED38D6DA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_BF3D1BFA89998DC5A7511933ED378535" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_6EBD60A9949E615AEC2B1933ED38D6DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6EBD60A9949E615AEC2B1933ED38D6DA" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_6EBD60A9949E615AEC2B1933ED38D6DA" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ThemisComputerAcquisitionMember" xlink:label="loc_mrcy_ThemisComputerAcquisitionMember_D89D934A1DAFA54EABDF1933ED3863B0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:to="loc_mrcy_ThemisComputerAcquisitionMember_D89D934A1DAFA54EABDF1933ED3863B0" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_RichlandTechnologiesLLCMember" xlink:label="loc_mrcy_RichlandTechnologiesLLCMember_A8D4E33500CEE3842AC81933ED3817E7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:to="loc_mrcy_RichlandTechnologiesLLCMember_A8D4E33500CEE3842AC81933ED3817E7" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_DeltaMicrowaveLLCMember" xlink:label="loc_mrcy_DeltaMicrowaveLLCMember_B4C9A0606D13CA2C53861933ED38747F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:to="loc_mrcy_DeltaMicrowaveLLCMember_B4C9A0606D13CA2C53861933ED38747F" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_CESCreativeElectronicSystemsS.A.Member" xlink:label="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_0B2CA47A6E40814742551933ED38E93B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_5EDB6A774F52374C1F9A1933ED388008" xlink:to="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_0B2CA47A6E40814742551933ED38E93B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_491A52793C47306A8EA11933ED3892C1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_BF3D1BFA89998DC5A7511933ED378535" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_491A52793C47306A8EA11933ED3892C1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_491A52793C47306A8EA11933ED3892C1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_491A52793C47306A8EA11933ED3892C1" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_0C7FF088751F85C120F01933ED381EE3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558" xlink:to="loc_us-gaap_CustomerRelationshipsMember_0C7FF088751F85C120F01933ED381EE3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_969FCECFDF27859C0B091933ED3890C8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_969FCECFDF27859C0B091933ED3890C8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OrderOrProductionBacklogMember" xlink:label="loc_us-gaap_OrderOrProductionBacklogMember_DE82E2587672D6ADF34E1933ED38E2D9" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_A8FEDAA29B343AEF6F1D1933ED387558" xlink:to="loc_us-gaap_OrderOrProductionBacklogMember_DE82E2587672D6ADF34E1933ED38E2D9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_CC20BB0D3F72C182A6D51933ED38E77C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_CC20BB0D3F72C182A6D51933ED38E77C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_6AAA03FF53D3159869A51933ED382218" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_6AAA03FF53D3159869A51933ED382218" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_F90120154FA6521CF9361933ED39C196" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_F90120154FA6521CF9361933ED39C196" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_DFEA102F9C0FC15AD7D31933ED397AE4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_DFEA102F9C0FC15AD7D31933ED397AE4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_765239FF7DA64844A0261933ED3936A9" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_Goodwill_765239FF7DA64844A0261933ED3936A9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_4E74E9D626CE436EA1441933ED39A7B0" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_4E74E9D626CE436EA1441933ED39A7B0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_26ED4686207125F860F81933ED394DD4" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationRevenueOfAcquireeSinceAcquisitionDateActual_26ED4686207125F860F81933ED394DD4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual" xlink:label="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_F0915F4DDACA724ECD001933ED39E9BF" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_BusinessCombinationProFormaInformationEarningsOrLossOfAcquireeSinceAcquisitionDateActual_F0915F4DDACA724ECD001933ED39E9BF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_864F4A8E1B4AFB832BC51933ED392582" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_9CFAD2A70F12F9B2D4621933ED386D4F" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_864F4A8E1B4AFB832BC51933ED392582" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/AcquisitionsNetPurchasePriceAndFairValuesOfAssetsAndLiabilitiesAcquiredDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C56DDD08D52444DF113D1933ED319259" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C56DDD08D52444DF113D1933ED319259" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_5E2B750A2D56DF5223A21933ED3156F6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_C56DDD08D52444DF113D1933ED319259" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_5E2B750A2D56DF5223A21933ED3156F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5E2B750A2D56DF5223A21933ED3156F6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_5E2B750A2D56DF5223A21933ED3156F6" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ThemisComputerAcquisitionMember" xlink:label="loc_mrcy_ThemisComputerAcquisitionMember_DBAE126EA82E0D594D751933ED310918" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB" xlink:to="loc_mrcy_ThemisComputerAcquisitionMember_DBAE126EA82E0D594D751933ED310918" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_DeltaMicrowaveLLCMember" xlink:label="loc_mrcy_DeltaMicrowaveLLCMember_FB505462DA7A10B831511933ED310EA5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB" xlink:to="loc_mrcy_DeltaMicrowaveLLCMember_FB505462DA7A10B831511933ED310EA5" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_CESCreativeElectronicSystemsS.A.Member" xlink:label="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_A2477AD6BA98EECCA1501933ED314B7C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_E57366E885465CECA1D41933ED31F8EB" xlink:to="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_A2477AD6BA98EECCA1501933ED314B7C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredAbstract" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_A604398FDFB7F503C6B71933ED32E149" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_A604398FDFB7F503C6B71933ED32E149" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_BusinessCombinationsConsiderationReceivedWorkingCapitalAdjustment" xlink:label="loc_mrcy_BusinessCombinationsConsiderationReceivedWorkingCapitalAdjustment_0EADDA777E612475DD8C1933ED328EF2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:to="loc_mrcy_BusinessCombinationsConsiderationReceivedWorkingCapitalAdjustment_0EADDA777E612475DD8C1933ED328EF2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CashAcquiredFromAcquisition" xlink:label="loc_us-gaap_CashAcquiredFromAcquisition_6026D413993926093FDD1933ED32D817" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:to="loc_us-gaap_CashAcquiredFromAcquisition_6026D413993926093FDD1933ED32D817" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_E196B60ACE73125A86261933ED32C2DC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationConsiderationTransferredAbstract_F806DE136F5F84C15B3F1933ED324D51" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_E196B60ACE73125A86261933ED32C2DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_17B0D6B7FBED0DF9A6B91933ED32105C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_17B0D6B7FBED0DF9A6B91933ED32105C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_ADD06D5720E0609037501933ED32F05D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_ADD06D5720E0609037501933ED32F05D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_BDA7726026C7287328141933ED32D907" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_BDA7726026C7287328141933ED32D907" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_0837745016322D3DAE131933ED32B47D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_0837745016322D3DAE131933ED32B47D" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsCurrentAndNoncurrent" xlink:label="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsCurrentAndNoncurrent_801B41AAD874BAEEC89C1933ED320A2B" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsCurrentAndNoncurrent_801B41AAD874BAEEC89C1933ED320A2B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_6ADCB0027CC3EBED26C11933ED32C47D" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_6ADCB0027CC3EBED26C11933ED32C47D" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAccruedLiabilities" xlink:label="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAccruedLiabilities_C9E9CE9B2C6E0C6FAAB41933ED3289C7" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAccruedLiabilities_C9E9CE9B2C6E0C6FAAB41933ED3289C7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_B7A649F60F6517CD335B1933ED325CC5" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_B7A649F60F6517CD335B1933ED325CC5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_49D709A5820F29271EC41933ED33C0AA" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_49D709A5820F29271EC41933ED33C0AA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_AD609B18EA35D9A3B54C1933ED3394F7" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilities_AD609B18EA35D9A3B54C1933ED3394F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent_F63705A74214F35260AF1933ED336755" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNetAbstract_2320E502613BFE0E30861933ED323AA8" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxLiabilitiesNoncurrent_F63705A74214F35260AF1933ED336755" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTangibleAssets" xlink:label="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTangibleAssets_77BFF27D9601CE2407761933ED33FE6B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_mrcy_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedTangibleAssets_77BFF27D9601CE2407761933ED33FE6B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_A8E09D942C5CBA1425521933ED332EEA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_A8E09D942C5CBA1425521933ED332EEA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_18642103BBD00588490C1933ED332424" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_Goodwill_18642103BBD00588490C1933ED332424" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_474893ACFE6D1B8131DC1933ED33BF6A" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B8DCF5B70ACA03AF4A511933ED31D2FB" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_474893ACFE6D1B8131DC1933ED33BF6A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/AcquisitionsProFormaDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C56377818C23AF06F4008DF63EC919F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C56377818C23AF06F4008DF63EC919F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0F2F8F17DCF097D5E46908DF63EC9FC4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_4C56377818C23AF06F4008DF63EC919F" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0F2F8F17DCF097D5E46908DF63EC9FC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64A7B20C6D061206CD1708DF63ED9C81_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0F2F8F17DCF097D5E46908DF63EC9FC4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64A7B20C6D061206CD1708DF63ED9C81_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64A7B20C6D061206CD1708DF63ED9C81" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0F2F8F17DCF097D5E46908DF63EC9FC4" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64A7B20C6D061206CD1708DF63ED9C81" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ThemisComputerAcquisitionMember" xlink:label="loc_mrcy_ThemisComputerAcquisitionMember_9B188DA229CE40EEABE608E20E048B61" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_64A7B20C6D061206CD1708DF63ED9C81" xlink:to="loc_mrcy_ThemisComputerAcquisitionMember_9B188DA229CE40EEABE608E20E048B61" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_FC6F0499D67E6DD47E7008DFFF30C6BB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_FC6F0499D67E6DD47E7008DFFF30C6BB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_725D7F1F6E1EE7AAFAA008E0698CE34F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_725D7F1F6E1EE7AAFAA008E0698CE34F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic_B849A8542F9154EDAFAF08E0E9ED25C2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:to="loc_us-gaap_BusinessAcquisitionProFormaEarningsPerShareBasic_B849A8542F9154EDAFAF08E0E9ED25C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted" xlink:label="loc_us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted_EE791ED8342E1BE52CA708E1524FC83C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_ABB9E9BF1AC5B20FAA2108DF63F34EEC" xlink:to="loc_us-gaap_BusinessAcquisitionProFormaEarningsPerShareDiluted_EE791ED8342E1BE52CA708E1524FC83C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/AcquisitionsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/ChangesInCarryingAmountOfGoodwillDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillLineItems" xlink:label="loc_us-gaap_GoodwillLineItems_40B3A5DF09EAE49F7D5B2724AA9E3D73" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfGoodwillTable" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_2DFA43C8EB37E53435812724AA8F7F3C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_40B3A5DF09EAE49F7D5B2724AA9E3D73" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_2DFA43C8EB37E53435812724AA8F7F3C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_2DDDFA72C8B994DD392D2724AA8F269A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_2DFA43C8EB37E53435812724AA8F7F3C" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_2DDDFA72C8B994DD392D2724AA8F269A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2DDDFA72C8B994DD392D2724AA8F269A" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_2DDDFA72C8B994DD392D2724AA8F269A" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_CESCreativeElectronicSystemsS.A.Member" xlink:label="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_85C940339F6EF5D17ECA2724AA8F902F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:to="loc_mrcy_CESCreativeElectronicSystemsS.A.Member_85C940339F6EF5D17ECA2724AA8F902F" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_DeltaMicrowaveLLCMember" xlink:label="loc_mrcy_DeltaMicrowaveLLCMember_DC7FD7C4F556492524DF2724AA8F313B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:to="loc_mrcy_DeltaMicrowaveLLCMember_DC7FD7C4F556492524DF2724AA8F313B" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_RichlandTechnologiesLLCMember" xlink:label="loc_mrcy_RichlandTechnologiesLLCMember_E66ED9AD5F17046C1F582724AA9E80B3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:to="loc_mrcy_RichlandTechnologiesLLCMember_E66ED9AD5F17046C1F582724AA9E80B3" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ThemisComputerAcquisitionMember" xlink:label="loc_mrcy_ThemisComputerAcquisitionMember_41B4AC01164E41F6CA6A2724AA9E6C97" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_04DE657B3FAEEA1557212724AA8F0E5B" xlink:to="loc_mrcy_ThemisComputerAcquisitionMember_41B4AC01164E41F6CA6A2724AA9E6C97" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementBusinessSegmentsAxis" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_DF63DE84986F97D917112724AA9E93D7" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_2DFA43C8EB37E53435812724AA8F7F3C" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_DF63DE84986F97D917112724AA9E93D7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_DF63DE84986F97D917112724AA9E93D7" xlink:to="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentDomain" xlink:label="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_DF63DE84986F97D917112724AA9E93D7" xlink:to="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_SensorAndMissionProcessingMember" xlink:label="loc_mrcy_SensorAndMissionProcessingMember_F6782761A96FA3DE990F2724ADC29D63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:to="loc_mrcy_SensorAndMissionProcessingMember_F6782761A96FA3DE990F2724ADC29D63" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_AdvancedMicroelectronicSolutionsMember" xlink:label="loc_mrcy_AdvancedMicroelectronicSolutionsMember_6F22D2F6E3E8E2A16A7E27256428840B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:to="loc_mrcy_AdvancedMicroelectronicSolutionsMember_6F22D2F6E3E8E2A16A7E27256428840B" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_CarveOutBusinessMember" xlink:label="loc_mrcy_CarveOutBusinessMember_37B75155E6BA3A761EA52724AA9E38F3" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:to="loc_mrcy_CarveOutBusinessMember_37B75155E6BA3A761EA52724AA9E38F3" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_MercuryDefenseSystemsMember" xlink:label="loc_mrcy_MercuryDefenseSystemsMember_5E00B593FE9EFDEF3A262725E042C606" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_23D725BFF2A5FC2FB6C12724AA9EC6A9" xlink:to="loc_mrcy_MercuryDefenseSystemsMember_5E00B593FE9EFDEF3A262725E042C606" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillRollForward" xlink:label="loc_us-gaap_GoodwillRollForward_74C854DCB9AFA50BBCCB2724AA9EE566" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_40B3A5DF09EAE49F7D5B2724AA9E3D73" xlink:to="loc_us-gaap_GoodwillRollForward_74C854DCB9AFA50BBCCB2724AA9EE566" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_133F9B3807989A5521822724AA9E51FA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_74C854DCB9AFA50BBCCB2724AA9EE566" xlink:to="loc_us-gaap_Goodwill_133F9B3807989A5521822724AA9E51FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_B0BF045E3CB7514B92A42724AA9E84A6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_74C854DCB9AFA50BBCCB2724AA9EE566" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_B0BF045E3CB7514B92A42724AA9E84A6" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/CommitmentsAndContingenciesAdditionalInformationDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermPurchaseCommitmentLineItems" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentLineItems_176E4101E13305B9654E7C3AF64DDACD" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermPurchaseCommitmentTable" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentTable_564BB55D7725928C3FE17C3AF64DC97F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_176E4101E13305B9654E7C3AF64DDACD" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentTable_564BB55D7725928C3FE17C3AF64DC97F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_57F57AA15AF2AAB998F97C3AF64DFAE5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentTable_564BB55D7725928C3FE17C3AF64DC97F" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_57F57AA15AF2AAB998F97C3AF64DFAE5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_409ACF0A8E97B67C08507C3AF64D5400_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_57F57AA15AF2AAB998F97C3AF64DFAE5" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_409ACF0A8E97B67C08507C3AF64D5400_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain" xlink:label="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_409ACF0A8E97B67C08507C3AF64D5400" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentByCategoryOfItemPurchasedAxis_57F57AA15AF2AAB998F97C3AF64DFAE5" xlink:to="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_409ACF0A8E97B67C08507C3AF64D5400" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_NonCancelablePurchaseCommitmentsMember" xlink:label="loc_mrcy_NonCancelablePurchaseCommitmentsMember_3DFB44CA5526553911007C3AF64D0B75" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentCategoryOfItemPurchasedDomain_409ACF0A8E97B67C08507C3AF64D5400" xlink:to="loc_mrcy_NonCancelablePurchaseCommitmentsMember_3DFB44CA5526553911007C3AF64D0B75" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted" xlink:label="loc_us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted_414FCB2BE8813324A8467C3AF64D7792" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermPurchaseCommitmentLineItems_176E4101E13305B9654E7C3AF64DDACD" xlink:to="loc_us-gaap_PurchaseCommitmentRemainingMinimumAmountCommitted_414FCB2BE8813324A8467C3AF64D7792" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/ConsolidatedBalanceSheets" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/ConsolidatedBalanceSheetsParenthetical" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/ConsolidatedStatementsOfCashFlows" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/ConsolidatedStatementsOfOperationsAndComprehensiveIncome" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/Debt" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/DebtAdditionalInformationDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentLineItems" xlink:label="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentTable" xlink:label="loc_us-gaap_DebtInstrumentTable_C452340AFACE8F116F23192E8E907841" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_DebtInstrumentTable_C452340AFACE8F116F23192E8E907841" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityAxis" xlink:label="loc_us-gaap_CreditFacilityAxis_6AFCA554CF135E77013C192E8E902449" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_C452340AFACE8F116F23192E8E907841" xlink:to="loc_us-gaap_CreditFacilityAxis_6AFCA554CF135E77013C192E8E902449" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_6AFCA554CF135E77013C192E8E902449" xlink:to="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CreditFacilityDomain" xlink:label="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_6AFCA554CF135E77013C192E8E902449" xlink:to="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RevolvingCreditFacilityMember" xlink:label="loc_us-gaap_RevolvingCreditFacilityMember_53DACD96F5DC40185BA6192E8E90D7B7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389" xlink:to="loc_us-gaap_RevolvingCreditFacilityMember_53DACD96F5DC40185BA6192E8E90D7B7" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_TermLoanMember" xlink:label="loc_mrcy_TermLoanMember_37AB75DF152D1E96F7A8192E8E907D93" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_51F3FA0B78EED493ABC8192E8E907389" xlink:to="loc_mrcy_TermLoanMember_37AB75DF152D1E96F7A8192E8E907D93" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_E3E72ECD79C4F99E6AAB192E8E909774" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_E3E72ECD79C4F99E6AAB192E8E909774" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DebtInstrumentTerm" xlink:label="loc_us-gaap_DebtInstrumentTerm_260FFAA3291C17D9E6D0192E8E906AC4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_DebtInstrumentTerm_260FFAA3291C17D9E6D0192E8E906AC4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DeferredFinanceCostsGross" xlink:label="loc_us-gaap_DeferredFinanceCostsGross_2B1B0477BED652B5E4ED192E8E91A50F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_DeferredFinanceCostsGross_2B1B0477BED652B5E4ED192E8E91A50F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProceedsFromLinesOfCredit" xlink:label="loc_us-gaap_ProceedsFromLinesOfCredit_91FB5625849794856C91192E8E912048" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_ProceedsFromLinesOfCredit_91FB5625849794856C91192E8E912048" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SecuredLongTermDebt" xlink:label="loc_us-gaap_SecuredLongTermDebt_93857D09A4813C4D8225193013722DF2" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_SecuredLongTermDebt_93857D09A4813C4D8225193013722DF2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_InterestExpense" xlink:label="loc_us-gaap_InterestExpense_F7D2740666F79623E4BF1933D50293A5" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_InterestExpense_F7D2740666F79623E4BF1933D50293A5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_LettersOfCreditOutstandingAmount" xlink:label="loc_us-gaap_LettersOfCreditOutstandingAmount_6F6DF8F02FAA6A0307D7192E8E916A38" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_BE7DD085AC1924174901192E8E907102" xlink:to="loc_us-gaap_LettersOfCreditOutstandingAmount_6F6DF8F02FAA6A0307D7192E8E916A38" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/DescriptionOfBusiness" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/DescriptionOfBusinessDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/EmployeeBenefitPlanDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanDisclosureLineItems" xlink:label="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable" xlink:label="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_DE125F264F97CFCE1F77192B3F7ACBC8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:to="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_DE125F264F97CFCE1F77192B3F7ACBC8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis" xlink:label="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_19C9B39CD649086967C8192B3F7AB679" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_DE125F264F97CFCE1F77192B3F7ACBC8" xlink:to="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_19C9B39CD649086967C8192B3F7AB679" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_F29AF67E1E07E1FBA3CC192B3F7ABC04_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_19C9B39CD649086967C8192B3F7AB679" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_F29AF67E1E07E1FBA3CC192B3F7ABC04_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansDomain" xlink:label="loc_us-gaap_DefinedBenefitPlansDomain_F29AF67E1E07E1FBA3CC192B3F7ABC04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_19C9B39CD649086967C8192B3F7AB679" xlink:to="loc_us-gaap_DefinedBenefitPlansDomain_F29AF67E1E07E1FBA3CC192B3F7ABC04" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PensionPlansDefinedBenefitMember" xlink:label="loc_us-gaap_PensionPlansDefinedBenefitMember_0C8CDFFCEB73D5829616192B3F7A9BA4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlansDomain_F29AF67E1E07E1FBA3CC192B3F7ABC04" xlink:to="loc_us-gaap_PensionPlansDefinedBenefitMember_0C8CDFFCEB73D5829616192B3F7A9BA4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_0B44E74FF903583C3439192B3F7BF6E3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfDefinedBenefitPlansDisclosuresTable_DE125F264F97CFCE1F77192B3F7ACBC8" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_0B44E74FF903583C3439192B3F7BF6E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_A7280223891A3B9C3FC1192B3F7BBB63_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_0B44E74FF903583C3439192B3F7BF6E3" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_A7280223891A3B9C3FC1192B3F7BBB63_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_A7280223891A3B9C3FC1192B3F7BBB63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_0B44E74FF903583C3439192B3F7BF6E3" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_A7280223891A3B9C3FC1192B3F7BBB63" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherNoncurrentLiabilitiesMember" xlink:label="loc_us-gaap_OtherNoncurrentLiabilitiesMember_EC2CBA37B36C9FF34672192B3F7B9765" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_A7280223891A3B9C3FC1192B3F7BBB63" xlink:to="loc_us-gaap_OtherNoncurrentLiabilitiesMember_EC2CBA37B36C9FF34672192B3F7B9765" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet" xlink:label="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet_0209DAE7FD3599E11C36192B3F7B5319" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:to="loc_us-gaap_DefinedBenefitPlanAmountsRecognizedInBalanceSheet_0209DAE7FD3599E11C36192B3F7B5319" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax_EA4E29528F31B1A71452192B3F7BC756" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossPensionAndOtherPostretirementBenefitPlansAdjustmentNetOfTax_EA4E29528F31B1A71452192B3F7BC756" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost" xlink:label="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_D2C9CF84D30B3313994D192B3F7BA657" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:to="loc_us-gaap_DefinedBenefitPlanNetPeriodicBenefitCost_D2C9CF84D30B3313994D192B3F7BA657" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear" xlink:label="loc_us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear_3BD474F27EF388A45362192B3F7B8E66" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DefinedBenefitPlanDisclosureLineItems_1A96A31CD5CC76D92EA3192B3F7BF607" xlink:to="loc_us-gaap_DefinedBenefitPlansEstimatedFutureEmployerContributionsInCurrentFiscalYear_3BD474F27EF388A45362192B3F7B8E66" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/EmployeeBenefitPlanEmployeeBenefitPlan" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/FairValueOfFinancialInstruments" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/FairValueOfFinancialInstrumentsSummaryOfFinancialAssetsMeasuredAtFairValueOnRecurringBasisDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_79BB2891C1D49445B1E57C3AF68944DB" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_79BB2891C1D49445B1E57C3AF68944DB" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_097748C20280EF789EF77C3AF67F0A15" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_097748C20280EF789EF77C3AF67F0A15" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_62546F1D7AB0316F12C07C3AF67F0DAF_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_097748C20280EF789EF77C3AF67F0A15" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_62546F1D7AB0316F12C07C3AF67F0DAF_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_8C310D86DBD523F20E317C3AF67F242D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_097748C20280EF789EF77C3AF67F0A15" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_8C310D86DBD523F20E317C3AF67F242D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_62546F1D7AB0316F12C07C3AF67F0DAF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_8C310D86DBD523F20E317C3AF67F242D" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_62546F1D7AB0316F12C07C3AF67F0DAF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_71E487F7E9769C9DA9A97C3AF67FD7C2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_8C310D86DBD523F20E317C3AF67F242D" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_71E487F7E9769C9DA9A97C3AF67FD7C2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_B1272E9E0A7B14BBCF127C3AF67F7198" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_B1272E9E0A7B14BBCF127C3AF67F7198" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_6F672D55DE4363AA46007C3AF689C141_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_B1272E9E0A7B14BBCF127C3AF67F7198" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_6F672D55DE4363AA46007C3AF689C141_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_6F672D55DE4363AA46007C3AF689C141" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_B1272E9E0A7B14BBCF127C3AF67F7198" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_6F672D55DE4363AA46007C3AF689C141" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_494B11EE26D4F2BCC6877C3AF68973C9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_6F672D55DE4363AA46007C3AF689C141" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_494B11EE26D4F2BCC6877C3AF68973C9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_33A6BB53421A6AD1B7F47C3AF689129C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_33A6BB53421A6AD1B7F47C3AF689129C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_33A6BB53421A6AD1B7F47C3AF689129C" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_33A6BB53421A6AD1B7F47C3AF689129C" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_30DD24FAF238265D3A6D7C3AF68901DA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_30DD24FAF238265D3A6D7C3AF68901DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_7E4692F3ADE2007DD0897C3AF689F6BA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_7E4692F3ADE2007DD0897C3AF689F6BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_81D78DBBDAB6D20F15807C3AF689855A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_791D810BE5CB00583D3B7C3AF68917DB" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_81D78DBBDAB6D20F15807C3AF689855A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueByAssetClassAxis" xlink:label="loc_us-gaap_FairValueByAssetClassAxis_780F2C61121D1B4CA8F27C3AF6894DBA" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_73BEDF861EC2264F46027C3AF67FEA47" xlink:to="loc_us-gaap_FairValueByAssetClassAxis_780F2C61121D1B4CA8F27C3AF6894DBA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_BEC0B637E592EE10DFBF7C3AF689A919_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_780F2C61121D1B4CA8F27C3AF6894DBA" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_BEC0B637E592EE10DFBF7C3AF689A919_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain" xlink:label="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_BEC0B637E592EE10DFBF7C3AF689A919" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByAssetClassAxis_780F2C61121D1B4CA8F27C3AF6894DBA" xlink:to="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_BEC0B637E592EE10DFBF7C3AF689A919" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="loc_us-gaap_CertificatesOfDepositMember_F0C3FF839E3AE584D73B7C3AF689743A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_BEC0B637E592EE10DFBF7C3AF689A919" xlink:to="loc_us-gaap_CertificatesOfDepositMember_F0C3FF839E3AE584D73B7C3AF689743A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AssetsFairValueDisclosureAbstract" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_900FB508397D65BFA04A7C3AF689C665" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_79BB2891C1D49445B1E57C3AF68944DB" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_900FB508397D65BFA04A7C3AF689C665" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_BC5FFE31A35CED2ECB917C3AF6898170" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_900FB508397D65BFA04A7C3AF689C665" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_BC5FFE31A35CED2ECB917C3AF6898170" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/FairValueOfFinancialInstrumentsTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/Goodwill" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/GoodwillTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/IncomeTaxes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/IncomeTaxesAdditionalInformationDetail" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/Inventory" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/InventoryAdditionalInformationDetail" xlink:type="extended">
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ScheduleOfInventoryLineItems" xlink:label="loc_mrcy_ScheduleOfInventoryLineItems_0FC4F2B0A451B4E617C67C3AF6A7E0C8" xlink:type="locator" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ScheduleOfInventoryTable" xlink:label="loc_mrcy_ScheduleOfInventoryTable_A148CEEAE9459572BCB27C3AF6A74201" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mrcy_ScheduleOfInventoryLineItems_0FC4F2B0A451B4E617C67C3AF6A7E0C8" xlink:to="loc_mrcy_ScheduleOfInventoryTable_A148CEEAE9459572BCB27C3AF6A74201" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_F641FFBB59F753A663BF7C3AF6A76E3A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mrcy_ScheduleOfInventoryTable_A148CEEAE9459572BCB27C3AF6A74201" xlink:to="loc_us-gaap_ProductOrServiceAxis_F641FFBB59F753A663BF7C3AF6A76E3A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_7279A07AB958F216B0C77C3AF6A71AD6_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_F641FFBB59F753A663BF7C3AF6A76E3A" xlink:to="loc_us-gaap_ProductsAndServicesDomain_7279A07AB958F216B0C77C3AF6A71AD6_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_7279A07AB958F216B0C77C3AF6A71AD6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_F641FFBB59F753A663BF7C3AF6A76E3A" xlink:to="loc_us-gaap_ProductsAndServicesDomain_7279A07AB958F216B0C77C3AF6A71AD6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductMember" xlink:label="loc_us-gaap_ProductMember_D1CA8449914FA1216CB37C3AF6A7B535" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_7279A07AB958F216B0C77C3AF6A71AD6" xlink:to="loc_us-gaap_ProductMember_D1CA8449914FA1216CB37C3AF6A7B535" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_InventoryForLongTermContractsOrPrograms" xlink:label="loc_us-gaap_InventoryForLongTermContractsOrPrograms_362E56723FA848B1622C7C3AF6A70C76" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_ScheduleOfInventoryLineItems_0FC4F2B0A451B4E617C67C3AF6A7E0C8" xlink:to="loc_us-gaap_InventoryForLongTermContractsOrPrograms_362E56723FA848B1622C7C3AF6A70C76" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_OperatingCycle" xlink:label="loc_us-gaap_OperatingCycle_A84AD6B669520B0BA2F67C3AF6A70191" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_ScheduleOfInventoryLineItems_0FC4F2B0A451B4E617C67C3AF6A7E0C8" xlink:to="loc_us-gaap_OperatingCycle_A84AD6B669520B0BA2F67C3AF6A70191" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/InventoryDetail" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/InventoryTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomers" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersCustomersComprisingTenPercentOrMoreRevenuesDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_B94414F7E3D2B9FBA8092737C16B2730" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_E3B8BBCEBDFB7F0E58252737C16B2F2E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_B94414F7E3D2B9FBA8092737C16B2730" xlink:to="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_E3B8BBCEBDFB7F0E58252737C16B2F2E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_3192B80D6C9C2E4513AD2737C16B02E8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_E3B8BBCEBDFB7F0E58252737C16B2F2E" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_3192B80D6C9C2E4513AD2737C16B02E8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_E0006C0D7D6D8BE5DEF22737C16B3952_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_3192B80D6C9C2E4513AD2737C16B02E8" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_E0006C0D7D6D8BE5DEF22737C16B3952_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_E0006C0D7D6D8BE5DEF22737C16B3952" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_3192B80D6C9C2E4513AD2737C16B02E8" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_E0006C0D7D6D8BE5DEF22737C16B3952" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_5C826D2346AA1F0A7B632737C16B18DF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_E0006C0D7D6D8BE5DEF22737C16B3952" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_5C826D2346AA1F0A7B632737C16B18DF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_54F370B2113BB8A17FD62737C16B1EEA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_E3B8BBCEBDFB7F0E58252737C16B2F2E" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_54F370B2113BB8A17FD62737C16B1EEA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_FC2091A45076742FA90A2737C16B62C3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_54F370B2113BB8A17FD62737C16B1EEA" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_FC2091A45076742FA90A2737C16B62C3_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_FC2091A45076742FA90A2737C16B62C3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_54F370B2113BB8A17FD62737C16B1EEA" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_FC2091A45076742FA90A2737C16B62C3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SalesRevenueNetMember" xlink:label="loc_us-gaap_SalesRevenueNetMember_11F8013FF4DE310473012737C16BC15D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_FC2091A45076742FA90A2737C16B62C3" xlink:to="loc_us-gaap_SalesRevenueNetMember_11F8013FF4DE310473012737C16BC15D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MajorCustomersAxis" xlink:label="loc_us-gaap_MajorCustomersAxis_4A428796D89F7CCC1E092737C16B7E5E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_E3B8BBCEBDFB7F0E58252737C16B2F2E" xlink:to="loc_us-gaap_MajorCustomersAxis_4A428796D89F7CCC1E092737C16B7E5E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorCustomersAxis_4A428796D89F7CCC1E092737C16B7E5E" xlink:to="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NameOfMajorCustomerDomain" xlink:label="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorCustomersAxis_4A428796D89F7CCC1E092737C16B7E5E" xlink:to="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_RaytheonCompanyMember" xlink:label="loc_mrcy_RaytheonCompanyMember_B6FC258B8E527E59AFE62737C16BBDA4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA" xlink:to="loc_mrcy_RaytheonCompanyMember_B6FC258B8E527E59AFE62737C16BBDA4" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_LockheedMartinCorporationMember" xlink:label="loc_mrcy_LockheedMartinCorporationMember_248E8767BBE8873DB8B327383E5AED92" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA" xlink:to="loc_mrcy_LockheedMartinCorporationMember_248E8767BBE8873DB8B327383E5AED92" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_NorthropGrummanCorporationMember" xlink:label="loc_mrcy_NorthropGrummanCorporationMember_0D83ADB6A0ED52B2CCC22737C16B913B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NameOfMajorCustomerDomain_709B52C4C2C86B34A9DE2737C16BEDDA" xlink:to="loc_mrcy_NorthropGrummanCorporationMember_0D83ADB6A0ED52B2CCC22737C16B913B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_0DE70AFA028D587F64882737C16BBD2E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_B94414F7E3D2B9FBA8092737C16B2730" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_0DE70AFA028D587F64882737C16BBD2E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersGeographicDistributionOfRevenuesAndLongLivedAssetsFromContinuingOperationsDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentReportingRevenueReconcilingItemLineItems" xlink:label="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_93074037AB98874C26C27C3AF69D6988" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable" xlink:label="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_93074037AB98874C26C27C3AF69D6988" xlink:to="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsAxis" xlink:label="loc_us-gaap_ConsolidationItemsAxis_72412BFAB73A79CC04EF7C3AF6931DEB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:to="loc_us-gaap_ConsolidationItemsAxis_72412BFAB73A79CC04EF7C3AF6931DEB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConsolidationItemsAxis_72412BFAB73A79CC04EF7C3AF6931DEB" xlink:to="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConsolidationItemsDomain" xlink:label="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConsolidationItemsAxis_72412BFAB73A79CC04EF7C3AF6931DEB" xlink:to="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ReportableGeographicalComponentsMember" xlink:label="loc_us-gaap_ReportableGeographicalComponentsMember_E12BB8CC385A1D6DBA9D7C3AF6933962" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA" xlink:to="loc_us-gaap_ReportableGeographicalComponentsMember_E12BB8CC385A1D6DBA9D7C3AF6933962" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographyEliminationsMember" xlink:label="loc_us-gaap_GeographyEliminationsMember_6ECD2F1F1AEE2BAA328C7C3AF6935B99" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConsolidationItemsDomain_3622082246CD7CAFD5117C3AF69380BA" xlink:to="loc_us-gaap_GeographyEliminationsMember_6ECD2F1F1AEE2BAA328C7C3AF6935B99" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_BE880372D50FA9A768DA7C3AF693C363" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:to="loc_us-gaap_StatementGeographicalAxis_BE880372D50FA9A768DA7C3AF693C363" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_BE880372D50FA9A768DA7C3AF693C363" xlink:to="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_BE880372D50FA9A768DA7C3AF693C363" xlink:to="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.sec.gov/country/2016/country-2016-01-31.xsd#country_US" xlink:label="loc_country_US_354478A5A5F3247B3D197C3AF6939555" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF" xlink:to="loc_country_US_354478A5A5F3247B3D197C3AF6939555" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EuropeMember" xlink:label="loc_us-gaap_EuropeMember_7D40427E34EB293E8C507C3AF6931C1A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF" xlink:to="loc_us-gaap_EuropeMember_7D40427E34EB293E8C507C3AF6931C1A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AsiaPacificMember" xlink:label="loc_us-gaap_AsiaPacificMember_8DAC8B31BAFCBB3ECB2F7C3AF693D423" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_EB1F04475DD9DE59EE6A7C3AF69335FF" xlink:to="loc_us-gaap_AsiaPacificMember_8DAC8B31BAFCBB3ECB2F7C3AF693D423" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographicDistributionAxis" xlink:label="loc_us-gaap_GeographicDistributionAxis_56340C045E652643D9BA7C3AF693CE0B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:to="loc_us-gaap_GeographicDistributionAxis_56340C045E652643D9BA7C3AF693CE0B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographicDistributionDomain" xlink:label="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GeographicDistributionAxis_56340C045E652643D9BA7C3AF693CE0B" xlink:to="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographicDistributionDomain" xlink:label="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GeographicDistributionAxis_56340C045E652643D9BA7C3AF693CE0B" xlink:to="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographicDistributionDomesticMember" xlink:label="loc_us-gaap_GeographicDistributionDomesticMember_EA475D744E06B41DFB197C3AF6939381" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC" xlink:to="loc_us-gaap_GeographicDistributionDomesticMember_EA475D744E06B41DFB197C3AF6939381" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_GeographicDistributionForeignMember" xlink:label="loc_us-gaap_GeographicDistributionForeignMember_5704C094B0D1AF04A7F07C3AF69D2B7D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GeographicDistributionDomain_7748568F2A126D4997777C3AF693A3BC" xlink:to="loc_us-gaap_GeographicDistributionForeignMember_5704C094B0D1AF04A7F07C3AF69D2B7D" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_EndApplicationAxis" xlink:label="loc_mrcy_EndApplicationAxis_15841B6D4F055BC477F27C3AF69D17C2" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:to="loc_mrcy_EndApplicationAxis_15841B6D4F055BC477F27C3AF69D17C2" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_EndApplicationsDomain" xlink:label="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mrcy_EndApplicationAxis_15841B6D4F055BC477F27C3AF69D17C2" xlink:to="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74_default" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_EndApplicationsDomain" xlink:label="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mrcy_EndApplicationAxis_15841B6D4F055BC477F27C3AF69D17C2" xlink:to="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_TotalSensorAndEffectorApplicationsMember" xlink:label="loc_mrcy_TotalSensorAndEffectorApplicationsMember_91F11B2CB0253328833B7C3AF69D5322" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74" xlink:to="loc_mrcy_TotalSensorAndEffectorApplicationsMember_91F11B2CB0253328833B7C3AF69D5322" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_RadarMember" xlink:label="loc_mrcy_RadarMember_DA0AE6C2BE215992A8427C3AF69D6C5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_TotalSensorAndEffectorApplicationsMember_91F11B2CB0253328833B7C3AF69D5322" xlink:to="loc_mrcy_RadarMember_DA0AE6C2BE215992A8427C3AF69D6C5B" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ElectronicWarfareMember" xlink:label="loc_mrcy_ElectronicWarfareMember_792DB1EF0E61B9203F927C3AF69D9CE9" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_TotalSensorAndEffectorApplicationsMember_91F11B2CB0253328833B7C3AF69D5322" xlink:to="loc_mrcy_ElectronicWarfareMember_792DB1EF0E61B9203F927C3AF69D9CE9" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_OtherSensorAndEffectorApplicationsMember" xlink:label="loc_mrcy_OtherSensorAndEffectorApplicationsMember_D02E521E0AEA1C9131C37C3AF69D176D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_TotalSensorAndEffectorApplicationsMember_91F11B2CB0253328833B7C3AF69D5322" xlink:to="loc_mrcy_OtherSensorAndEffectorApplicationsMember_D02E521E0AEA1C9131C37C3AF69D176D" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_C4IApplicationsMember" xlink:label="loc_mrcy_C4IApplicationsMember_5468D45276773FE2309B7C3AF69DBF97" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74" xlink:to="loc_mrcy_C4IApplicationsMember_5468D45276773FE2309B7C3AF69DBF97" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_OtherEndApplicationsMember" xlink:label="loc_mrcy_OtherEndApplicationsMember_E5359514F2927EF816CC7C3AF69D46D5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_EndApplicationsDomain_242038FCA0B53BB472727C3AF69D1E74" xlink:to="loc_mrcy_OtherEndApplicationsMember_E5359514F2927EF816CC7C3AF69D46D5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_E4E0782B958F299617907C3AF69DFB00" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfRevenueFromSegmentsToConsolidatedTable_3514AC5E77249AA6E62E7C3AF693A4D5" xlink:to="loc_us-gaap_ProductOrServiceAxis_E4E0782B958F299617907C3AF69DFB00" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_E4E0782B958F299617907C3AF69DFB00" xlink:to="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_E4E0782B958F299617907C3AF69DFB00" xlink:to="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ComponentsMember" xlink:label="loc_mrcy_ComponentsMember_D205813FDB85DD14FE1C7C3AF69D8A92" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB" xlink:to="loc_mrcy_ComponentsMember_D205813FDB85DD14FE1C7C3AF69D8A92" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ModulesandSubassembliesMember" xlink:label="loc_mrcy_ModulesandSubassembliesMember_938B98C41FAE6B7C2AFD7C3AF69D6EC6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB" xlink:to="loc_mrcy_ModulesandSubassembliesMember_938B98C41FAE6B7C2AFD7C3AF69D6EC6" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_IntegratedSubsystemsMember" xlink:label="loc_mrcy_IntegratedSubsystemsMember_E118DF3BDCCDBBF6CCED7C3AF69D3570" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_A05CF926DCCFCA8B54687C3AF69D84AB" xlink:to="loc_mrcy_IntegratedSubsystemsMember_E118DF3BDCCDBBF6CCED7C3AF69D3570" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_Revenues" xlink:label="loc_us-gaap_Revenues_F9DB4F1DD130A79933777C3AF69D60EF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_93074037AB98874C26C27C3AF69D6988" xlink:to="loc_us-gaap_Revenues_F9DB4F1DD130A79933777C3AF69D60EF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_NoncurrentAssets" xlink:label="loc_us-gaap_NoncurrentAssets_EF3DD2456165725D38A27C3AF69D6F3E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingRevenueReconcilingItemLineItems_93074037AB98874C26C27C3AF69D6988" xlink:to="loc_us-gaap_NoncurrentAssets_EF3DD2456165725D38A27C3AF69D6F3E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersProgramsComprisingTenPercentOrMoreOfCompanysRevenueDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_236ADDCB7178A4FF02931933EC474C94" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_6BE9D1114AF355A916BC1933EC440A57" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_236ADDCB7178A4FF02931933EC474C94" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_6BE9D1114AF355A916BC1933EC440A57" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_C7E068013BFFCA7C8D151933EC45CC89" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_6BE9D1114AF355A916BC1933EC440A57" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_C7E068013BFFCA7C8D151933EC45CC89" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_09A855162BE4211D35591933EC455EE7_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_C7E068013BFFCA7C8D151933EC45CC89" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_09A855162BE4211D35591933EC455EE7_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_09A855162BE4211D35591933EC455EE7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_C7E068013BFFCA7C8D151933EC45CC89" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_09A855162BE4211D35591933EC455EE7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SalesRevenueNetMember" xlink:label="loc_us-gaap_SalesRevenueNetMember_56565ACCDE1E5368E9521933EC45AB9A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_09A855162BE4211D35591933EC455EE7" xlink:to="loc_us-gaap_SalesRevenueNetMember_56565ACCDE1E5368E9521933EC45AB9A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_2DFEB6B0B231A91E99DA1933EC45D7DA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_6BE9D1114AF355A916BC1933EC440A57" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_2DFEB6B0B231A91E99DA1933EC45D7DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_EEFD3CF2E54C4E91D0661933EC45A43F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_2DFEB6B0B231A91E99DA1933EC45D7DA" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_EEFD3CF2E54C4E91D0661933EC45A43F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_EEFD3CF2E54C4E91D0661933EC45A43F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_2DFEB6B0B231A91E99DA1933EC45D7DA" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_EEFD3CF2E54C4E91D0661933EC45A43F" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ProgramConcentrationRiskMember" xlink:label="loc_mrcy_ProgramConcentrationRiskMember_DB75FBC635CC33565DDD1933EC4515CA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_EEFD3CF2E54C4E91D0661933EC45A43F" xlink:to="loc_mrcy_ProgramConcentrationRiskMember_DB75FBC635CC33565DDD1933EC4515CA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="loc_us-gaap_ProductOrServiceAxis_B2D4F72431C8E7FB1C331933EC45882C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_6BE9D1114AF355A916BC1933EC440A57" xlink:to="loc_us-gaap_ProductOrServiceAxis_B2D4F72431C8E7FB1C331933EC45882C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_476725D4A490682ECA221933EC45A7AB_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ProductOrServiceAxis_B2D4F72431C8E7FB1C331933EC45882C" xlink:to="loc_us-gaap_ProductsAndServicesDomain_476725D4A490682ECA221933EC45A7AB_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="loc_us-gaap_ProductsAndServicesDomain_476725D4A490682ECA221933EC45A7AB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ProductOrServiceAxis_B2D4F72431C8E7FB1C331933EC45882C" xlink:to="loc_us-gaap_ProductsAndServicesDomain_476725D4A490682ECA221933EC45A7AB" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_SEWIPProgramMember" xlink:label="loc_mrcy_SEWIPProgramMember_388611FCB983B3B30DAC196921727F5A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductsAndServicesDomain_476725D4A490682ECA221933EC45A7AB" xlink:to="loc_mrcy_SEWIPProgramMember_388611FCB983B3B30DAC196921727F5A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_2A102B898020CB780CAC1933EC474277" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_236ADDCB7178A4FF02931933EC474C94" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_2A102B898020CB780CAC1933EC474277" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/OperatingSegmentGeographicInformationAndSignificantCustomersTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/Restructuring" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/RestructuringAdditionalInformationDetail" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/RestructuringExpensesByReportableSegmentForRestructuringPlansDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_08E6565C8A79C14241431933ECE58A00" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_DA6962E69FD2F6292E441933ECE5C0F0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_08E6565C8A79C14241431933ECE58A00" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_DA6962E69FD2F6292E441933ECE5C0F0" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_B899F767E6D0006654FC1933ECE5A98B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_DA6962E69FD2F6292E441933ECE5C0F0" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_B899F767E6D0006654FC1933ECE5A98B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_B899F767E6D0006654FC1933ECE5A98B" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_B899F767E6D0006654FC1933ECE5A98B" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_8234A85DCECB557E85B31933ECE54D04" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A" xlink:to="loc_us-gaap_EmployeeSeveranceMember_8234A85DCECB557E85B31933ECE54D04" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_FacilityClosingMember" xlink:label="loc_us-gaap_FacilityClosingMember_2A38B2831613B6A7EE9C1933ECE58E51" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_89FAF5721344F9A52DA81933ECE5064A" xlink:to="loc_us-gaap_FacilityClosingMember_2A38B2831613B6A7EE9C1933ECE58E51" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveRollForward" xlink:label="loc_us-gaap_RestructuringReserveRollForward_6BF7A8FE6DA3D2FE24431933ECE58157" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_08E6565C8A79C14241431933ECE58A00" xlink:to="loc_us-gaap_RestructuringReserveRollForward_6BF7A8FE6DA3D2FE24431933ECE58157" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringReserveCurrent" xlink:label="loc_us-gaap_RestructuringReserveCurrent_8FE2B130A022E6E97D841933ECE51755" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_6BF7A8FE6DA3D2FE24431933ECE58157" xlink:to="loc_us-gaap_RestructuringReserveCurrent_8FE2B130A022E6E97D841933ECE51755" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestructuringSettlementAndImpairmentProvisions" xlink:label="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_12117103D569CA84DE1F1933ECE5CE84" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_6BF7A8FE6DA3D2FE24431933ECE58157" xlink:to="loc_us-gaap_RestructuringSettlementAndImpairmentProvisions_12117103D569CA84DE1F1933ECE5CE84" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PaymentsForRestructuring" xlink:label="loc_us-gaap_PaymentsForRestructuring_86AE140EF797D9ADC35A1933ECE5C1C5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringReserveRollForward_6BF7A8FE6DA3D2FE24431933ECE58157" xlink:to="loc_us-gaap_PaymentsForRestructuring_86AE140EF797D9ADC35A1933ECE5C1C5" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/RestructuringTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensationAdditionalInformationDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5DE4FE98A5AB9939B17F7C3AF6753C23" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5DE4FE98A5AB9939B17F7C3AF6753C23" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="loc_us-gaap_PlanNameAxis_434A789859BE779CE4B57C3AF675B740" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5DE4FE98A5AB9939B17F7C3AF6753C23" xlink:to="loc_us-gaap_PlanNameAxis_434A789859BE779CE4B57C3AF675B740" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_44080588299D1A57D3E07C3AF675FA29_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_434A789859BE779CE4B57C3AF675B740" xlink:to="loc_us-gaap_PlanNameDomain_44080588299D1A57D3E07C3AF675FA29_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="loc_us-gaap_PlanNameDomain_44080588299D1A57D3E07C3AF675FA29" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_434A789859BE779CE4B57C3AF675B740" xlink:to="loc_us-gaap_PlanNameDomain_44080588299D1A57D3E07C3AF675FA29" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_StockOptionPlanTwentyZeroFiveMember" xlink:label="loc_mrcy_StockOptionPlanTwentyZeroFiveMember_FA15A40B528FFE4B80757C3AF675B0DD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_44080588299D1A57D3E07C3AF675FA29" xlink:to="loc_mrcy_StockOptionPlanTwentyZeroFiveMember_FA15A40B528FFE4B80757C3AF675B0DD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_3D19BAE20821720054497C3AF67571FF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5DE4FE98A5AB9939B17F7C3AF6753C23" xlink:to="loc_us-gaap_RangeAxis_3D19BAE20821720054497C3AF67571FF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_EF20396953C1D6DC62767C3AF6751BA5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_3D19BAE20821720054497C3AF67571FF" xlink:to="loc_us-gaap_RangeMember_EF20396953C1D6DC62767C3AF6751BA5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_EF20396953C1D6DC62767C3AF6751BA5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_3D19BAE20821720054497C3AF67571FF" xlink:to="loc_us-gaap_RangeMember_EF20396953C1D6DC62767C3AF6751BA5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_089A5B72269DE9065F287C3AF675DBA5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_EF20396953C1D6DC62767C3AF6751BA5" xlink:to="loc_us-gaap_MaximumMember_089A5B72269DE9065F287C3AF675DBA5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_B49BFF4DC98BD5FF375A7C3AF675873A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5DE4FE98A5AB9939B17F7C3AF6753C23" xlink:to="loc_us-gaap_AwardTypeAxis_B49BFF4DC98BD5FF375A7C3AF675873A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F4E4F7A54D092CC79EB47C3AF67534C1_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_B49BFF4DC98BD5FF375A7C3AF675873A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F4E4F7A54D092CC79EB47C3AF67534C1_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F4E4F7A54D092CC79EB47C3AF67534C1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_B49BFF4DC98BD5FF375A7C3AF675873A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F4E4F7A54D092CC79EB47C3AF67534C1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_526B891CF626C01534287C3AF6754BE3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F4E4F7A54D092CC79EB47C3AF67534C1" xlink:to="loc_us-gaap_EmployeeStockMember_526B891CF626C01534287C3AF6754BE3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_D5BDEA3357CB44BAB65C7C3AF675B810" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_D5BDEA3357CB44BAB65C7C3AF675B810" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_MinimumGrantDateFairValueOfNonqualifiedStockOptionsAsPercentageOfFairValueOfCommonStock" xlink:label="loc_mrcy_MinimumGrantDateFairValueOfNonqualifiedStockOptionsAsPercentageOfFairValueOfCommonStock_139AB00B6DBED57B54767C3AF675CD76" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_mrcy_MinimumGrantDateFairValueOfNonqualifiedStockOptionsAsPercentageOfFairValueOfCommonStock_139AB00B6DBED57B54767C3AF675CD76" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardContractualLife" xlink:label="loc_mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardContractualLife_2398C7F0801488C37F1F7C3AF6751ACF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardContractualLife_2398C7F0801488C37F1F7C3AF6751ACF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_C476ADE5BCA0AA7ED78E7C3AF6756E64" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_C476ADE5BCA0AA7ED78E7C3AF6756E64" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate_5FED998766BD9FA013A07C3AF675AAB6" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate_5FED998766BD9FA013A07C3AF675AAB6" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_EmployeeStockPurchasePlanMaximumPercentageOfCompensationToPurchaseSharesByEligibleParticipants" xlink:label="loc_mrcy_EmployeeStockPurchasePlanMaximumPercentageOfCompensationToPurchaseSharesByEligibleParticipants_DCC7CEFA614BC660CAAB7C3AF67518E0" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_mrcy_EmployeeStockPurchasePlanMaximumPercentageOfCompensationToPurchaseSharesByEligibleParticipants_DCC7CEFA614BC660CAAB7C3AF67518E0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_0EF7C150896F19D79A647C3AF675CB2C" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_0EF7C150896F19D79A647C3AF675CB2C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_1925942A181EB292929F7C3AF6752B8A" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_429F5C33CE0D028D44D47C3AF675AF76" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_1925942A181EB292929F7C3AF6752B8A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensationStockBasedCompensationExpensesDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7F545F68D42F01F753F27C3AF6D9213C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_62033FAE81235467F41F7C3AF6D925A1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7F545F68D42F01F753F27C3AF6D9213C" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_62033FAE81235467F41F7C3AF6D925A1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_61298305C7112094D2597C3AF6D9DF4B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_62033FAE81235467F41F7C3AF6D925A1" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_61298305C7112094D2597C3AF6D9DF4B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_61298305C7112094D2597C3AF6D9DF4B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_61298305C7112094D2597C3AF6D9DF4B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_CostOfSalesMember" xlink:label="loc_us-gaap_CostOfSalesMember_A6798907FD649A553CCF7C3AF6D99100" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A" xlink:to="loc_us-gaap_CostOfSalesMember_A6798907FD649A553CCF7C3AF6D99100" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_4B31323D28848D4AD5AF7C3AF6D958A2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_4B31323D28848D4AD5AF7C3AF6D958A2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_8344F9EC4955D8C93AE97C3AF6D9668D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_80996CDB1869678049137C3AF6D94C2A" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_8344F9EC4955D8C93AE97C3AF6D9668D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_AB9876F2BDAA5A8BDD9D7C3AF6D99483" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7F545F68D42F01F753F27C3AF6D9213C" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_AB9876F2BDAA5A8BDD9D7C3AF6D99483" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_0EC277577DE2F8F3102C7C3AF6D9941E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7F545F68D42F01F753F27C3AF6D9213C" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_0EC277577DE2F8F3102C7C3AF6D9941E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_9049B33973065C035A637C3AF6D9C2B1" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_7F545F68D42F01F753F27C3AF6D9213C" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpenseNetOfTax_9049B33973065C035A637C3AF6D9C2B1" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensationSummaryOfNonvestedRestrictedStockDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F205AF1E55557F7CC0547C3AF6BBDF6C" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F0D2B61CCE784F9600D7C3AF6BB577B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F205AF1E55557F7CC0547C3AF6BBDF6C" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F0D2B61CCE784F9600D7C3AF6BB577B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_7B62767D3B195325DD287C3AF6BB20CA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_5F0D2B61CCE784F9600D7C3AF6BB577B" xlink:to="loc_us-gaap_AwardTypeAxis_7B62767D3B195325DD287C3AF6BB20CA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E29BF2376147E33B0F2A7C3AF6BB58F3_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_7B62767D3B195325DD287C3AF6BB20CA" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E29BF2376147E33B0F2A7C3AF6BB58F3_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E29BF2376147E33B0F2A7C3AF6BB58F3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_7B62767D3B195325DD287C3AF6BB20CA" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E29BF2376147E33B0F2A7C3AF6BB58F3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="loc_us-gaap_RestrictedStockMember_E29E16CB885F816B12F17C3AF6BB8EAA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E29BF2376147E33B0F2A7C3AF6BB58F3" xlink:to="loc_us-gaap_RestrictedStockMember_E29E16CB885F816B12F17C3AF6BB8EAA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F205AF1E55557F7CC0547C3AF6BBDF6C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_69D735DEA18B2CA4EEE77C3AF6BBF894" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_69D735DEA18B2CA4EEE77C3AF6BBF894" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4BE78188BE73DC7154217C3AF6BB603C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_4BE78188BE73DC7154217C3AF6BB603C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_C3228DB98B37D9A20ADA7C3AF6BBD6BF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_C3228DB98B37D9A20ADA7C3AF6BBD6BF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_40C67D1259673920514E7C3AF6BBC934" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_4645DF4025D5C4A8B4DB7C3AF6BB6780" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_40C67D1259673920514E7C3AF6BBC934" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F205AF1E55557F7CC0547C3AF6BBDF6C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_69BD2C75F45A0C04D3BE7C3AF6BBCC1E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_69BD2C75F45A0C04D3BE7C3AF6BBCC1E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_300B20A4D414DB9B562C7C3AF6BB2B41" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_300B20A4D414DB9B562C7C3AF6BB2B41" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_396B0E5577AA01CFCA467C3AF6BB6B05" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_396B0E5577AA01CFCA467C3AF6BB6B05" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_B0E6E96F3AB3B0C5A5017C3AF6BBC9B9" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_CB9CB6440FC737420EB27C3AF6BB3D12" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_B0E6E96F3AB3B0C5A5017C3AF6BBC9B9" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensationSummaryOfStockOptionPlansDetail" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F5BDB835A908637CC6367C3AF701D199" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4E1927C456546BBCB6057C3AF701FDE8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F5BDB835A908637CC6367C3AF701D199" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4E1927C456546BBCB6057C3AF701FDE8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_2E2DB4EE7BC8A7D012D37C3AF701BB5A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4E1927C456546BBCB6057C3AF701FDE8" xlink:to="loc_us-gaap_AwardTypeAxis_2E2DB4EE7BC8A7D012D37C3AF701BB5A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_CEE251BB1FAD95392DA87C3AF701B78C_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2E2DB4EE7BC8A7D012D37C3AF701BB5A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_CEE251BB1FAD95392DA87C3AF701B78C_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_CEE251BB1FAD95392DA87C3AF701B78C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2E2DB4EE7BC8A7D012D37C3AF701BB5A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_CEE251BB1FAD95392DA87C3AF701B78C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_0486D332197C2237CC327C3AF701B90B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_CEE251BB1FAD95392DA87C3AF701B78C" xlink:to="loc_us-gaap_EmployeeStockOptionMember_0486D332197C2237CC327C3AF701B90B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F5BDB835A908637CC6367C3AF701D199" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_2B9C660425881B6DE7977C3AF7016881" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_2B9C660425881B6DE7977C3AF7016881" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_4911AADE13B77A4DF7B37C3AF7010CAD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_4911AADE13B77A4DF7B37C3AF7010CAD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_D9BFCE2FAA280369C0B67C3AF701F533" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_D9BFCE2FAA280369C0B67C3AF701F533" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_DE0272123FE7D63860577C3AF7014583" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_BD9AF4E35DB79A9483227C3AF701FE0F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod_DE0272123FE7D63860577C3AF7014583" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F5BDB835A908637CC6367C3AF701D199" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_A1C8ABDAED7DABD7D1507C3AF701BC2C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_A1C8ABDAED7DABD7D1507C3AF701BC2C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_AA89150C6DA4E224D21D7C3AF7011D88" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_AA89150C6DA4E224D21D7C3AF7011D88" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_F799AFE7088BEAE239217C3AF701373D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_F799AFE7088BEAE239217C3AF701373D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_67379617CC0902E92F407C3AF70178D3" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_057B7FF004A7D78D48927C3AF7016244" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice_67379617CC0902E92F407C3AF70178D3" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract" xlink:label="loc_mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_1342139978B77C51CD897C3AF701C862" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_F5BDB835A908637CC6367C3AF701D199" xlink:to="loc_mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_1342139978B77C51CD897C3AF701C862" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_544F1B16781EB9A881277C3AF7013516" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsWeightedAverageRemainingContractualTermAbstract_1342139978B77C51CD897C3AF701C862" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_544F1B16781EB9A881277C3AF7013516" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/StockBasedCompensationTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SubsequentEvents" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesAdditionalInformationDetail" xlink:type="extended">
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_SignificantAccountingPoliciesLineItems" xlink:label="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:type="locator" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_SignificantAccountingPoliciesTable" xlink:label="loc_mrcy_SignificantAccountingPoliciesTable_75CED90CBC97EF90997719B6F64FC879" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_SignificantAccountingPoliciesTable_75CED90CBC97EF90997719B6F64FC879" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeAxis_96589076C5C072EA507F19B6F64F2794" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mrcy_SignificantAccountingPoliciesTable_75CED90CBC97EF90997719B6F64FC879" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeAxis_96589076C5C072EA507F19B6F64F2794" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_96589076C5C072EA507F19B6F64F2794" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2016/elts/us-gaap-2016-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeAxis_96589076C5C072EA507F19B6F64F2794" xlink:to="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ShipandBillMember" xlink:label="loc_mrcy_ShipandBillMember_A8C3C3E4EB7DFEAC50B019B6F65077FB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC" xlink:to="loc_mrcy_ShipandBillMember_A8C3C3E4EB7DFEAC50B019B6F65077FB" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_MultipleDeliveryRevenueMember" xlink:label="loc_mrcy_MultipleDeliveryRevenueMember_D1DE618691EC0952F6CD19B6F6504344" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC" xlink:to="loc_mrcy_MultipleDeliveryRevenueMember_D1DE618691EC0952F6CD19B6F6504344" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_ContractAccountingMember" xlink:label="loc_mrcy_ContractAccountingMember_9A619B768836AB2DC82119B6F6501BA0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredRevenueArrangementTypeDomain_A5B8D23EE990074FCF9619B6F64F47AC" xlink:to="loc_mrcy_ContractAccountingMember_9A619B768836AB2DC82119B6F6501BA0" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_RevenueRecognitionNumberOfAccountingMethods" xlink:label="loc_mrcy_RevenueRecognitionNumberOfAccountingMethods_89A311F7B06D36F38C8119B7BCE7AF0E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_RevenueRecognitionNumberOfAccountingMethods_89A311F7B06D36F38C8119B7BCE7AF0E" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_AccountsReceivablePurchaseAgreementMaximumReceivableBalances" xlink:label="loc_mrcy_AccountsReceivablePurchaseAgreementMaximumReceivableBalances_3037A6F99D716258181A19B6F650D2FE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_AccountsReceivablePurchaseAgreementMaximumReceivableBalances_3037A6F99D716258181A19B6F650D2FE" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_AccountsReceivableFactored" xlink:label="loc_mrcy_AccountsReceivableFactored_306BB7F87B3BF8ED818F19B6F6500593" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_AccountsReceivableFactored_306BB7F87B3BF8ED818F19B6F6500593" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_AccountsReceivableFactoringFees" xlink:label="loc_mrcy_AccountsReceivableFactoringFees_96F1171F56C281B3BA6F19B6F650A8B5" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_AccountsReceivableFactoringFees_96F1171F56C281B3BA6F19B6F650A8B5" xlink:type="arc" />
    <link:loc xlink:href="mrcy-20180331.xsd#mrcy_PercentageOfRevenueRecognized" xlink:label="loc_mrcy_PercentageOfRevenueRecognized_3FE341C4144C9853EAB619B6F65038E2" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mrcy_SignificantAccountingPoliciesLineItems_8387EC285C68101CB72719B6F6502D19" xlink:to="loc_mrcy_PercentageOfRevenueRecognized_3FE341C4144C9853EAB619B6F65038E2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesBasicAndDilutedWeightedAverageSharesOutstandingDetail" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.mrcy.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:type="extended" />
</link:linkbase>
