<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0203</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2010-06-28</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000701811</issuerCik>
        <issuerName>MENTOR GRAPHICS CORP</issuerName>
        <issuerTradingSymbol>MENT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000921669</rptOwnerCik>
            <rptOwnerName>ICAHN CARL C</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O ICAHN ASSOCIATES CORP.</rptOwnerStreet1>
            <rptOwnerStreet2>767 FIFTH AVE., SUITE 4700</rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10153</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>1</isOther>
            <officerTitle></officerTitle>
            <otherText>please see all footnotes</otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000928464</rptOwnerCik>
            <rptOwnerName>HIGH RIVER LIMITED PARTNERSHIP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001322827</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GEORGE TOWN, GRAND CAYMAN</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001414181</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND II L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN, CAYMAN ISLANDS</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001414179</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS MASTER FUND III L.P.</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O WALKER HOUSE</rptOwnerStreet1>
            <rptOwnerStreet2>87 MARY STREET</rptOwnerStreet2>
            <rptOwnerCity>GRAND CAYMAN, CAYMAN ISLANDS</rptOwnerCity>
            <rptOwnerState>E9</rptOwnerState>
            <rptOwnerZipCode>KY1-9001</rptOwnerZipCode>
            <rptOwnerStateDescription>CAYMAN ISLANDS</rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001313666</rptOwnerCik>
            <rptOwnerName>ICAHN PARTNERS LP</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>445 HAMILTON AVENUE</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 1210</rptOwnerStreet2>
            <rptOwnerCity>WHITE PLAINS</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10601</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>1</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock, without par value (&quot;Shares&quot;)</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>11582396</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see all footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Call Option (&quot;right to purchase&quot;)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>5.65</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-06-07</value>
            </exerciseDate>
            <expirationDate>
                <value>2012-06-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Shares</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3813573</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see all footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Put Option (&quot;obligation to purchase&quot;)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>5.65</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <value>2010-06-07</value>
            </exerciseDate>
            <expirationDate>
                <value>2012-06-07</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>please see footnote</value>
                    <footnoteId id="F8"/>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>3813573</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>please see all footnotes</value>
                    <footnoteId id="F1"/>
                    <footnoteId id="F2"/>
                    <footnoteId id="F3"/>
                    <footnoteId id="F4"/>
                    <footnoteId id="F5"/>
                    <footnoteId id="F6"/>
                    <footnoteId id="F7"/>
                    <footnoteId id="F8"/>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">High River Limited Partnership (&quot;High River&quot;) directly beneficially owns 2,316,479 Shares (including shares underlying Call Options (as defined below)), Icahn Partners LP (&quot;Icahn Partners&quot;) directly beneficially owns 3,371,520 Shares (including shares underlying Call Options), Icahn Partners Master Fund LP (&quot;Icahn Master&quot;) directly beneficially owns 3,976,671 Shares (including shares underlying Call Options), Icahn Partners Master Fund II LP (&quot;Icahn Master II&quot;) directly beneficially owns 1,392,042 Shares (including shares underlying Call Options), and Icahn Partners Master Fund III LP (&quot;Icahn Master III&quot;) directly beneficially owns 525,684 Shares (including shares underlying Call Options).</footnote>
        <footnote id="F2">Barberry Corp. (&quot;Barberry&quot;), is the sole member of Hopper Investments LLC (&quot;Hopper&quot;), which is the general partner of High River. Beckton Corp. (&quot;Beckton&quot;) is the sole stockholder of Icahn Enterprises G.P. Inc. (&quot;Icahn Enterprises GP&quot;), which is the general partner of Icahn Enterprises Holdings L.P. (&quot;Icahn Enterprises Holdings&quot;). Icahn Enterprises Holdings is the sole member of IPH GP LLC (&quot;IPH&quot;), which is the general partner of Icahn Capital LP (&quot;Icahn Capital&quot;). Icahn Capital is the general partner of each of Icahn Onshore LP (&quot;Icahn Onshore&quot;) and Icahn Offshore LP (&quot;Icahn Offshore&quot;). Icahn Onshore is the general partner of Icahn Partners. Icahn Offshore is the general partner of each of Icahn Master, Icahn Master II and Icahn Master III.</footnote>
        <footnote id="F3">Each of Barberry and Beckton is 100 percent owned by Carl C. Icahn. As such, Mr. Icahn is in a position indirectly to determine the investment and voting decisions made by each of High River, Icahn Partners, Icahn Master, Icahn Master II and Icahn Master III. Each of Hopper, Barberry and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares (including shares underlying Call Options) which High River owns. Each of Hopper, Barberry and Mr. Icahn disclaims beneficial ownership of such Shares (including shares underlying Call Options) except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F4">Each of Icahn Onshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares (including shares underlying Call Options) which Icahn Partners owns. Each of Icahn Onshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn disclaims beneficial ownership of such Shares (including shares underlying Call Options) except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F5">Each of Icahn Offshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn may be deemed to indirectly beneficially own (as that term is defined in Rule 13d-3 under the Act) the Shares (including shares underlying Call Options) which each of Icahn Master, Icahn Master II and Icahn Master III owns. Each of Icahn Offshore, Icahn Capital, IPH, Icahn Enterprises Holdings, Icahn Enterprises GP, Beckton and Mr. Icahn disclaims beneficial ownership of such Shares (including shares underlying Call Options) except to the extent of their pecuniary interest therein.</footnote>
        <footnote id="F6">High River directly beneficially owns 762,715 call options (the &quot;Call Options&quot;), Icahn Partners directly beneficially owns 1,110,093 Call Options, Icahn Master directly beneficially owns 1,309,342 Call Options, Icahn Master II directly beneficially owns 458,338 Call Options, and Icahn Master III directly beneficially owns 173,085 Call Options.</footnote>
        <footnote id="F7">High River directly wrote 762,715 European-style put options (the &quot;Put Options&quot;), Icahn Partners directly wrote 1,110,093 Put Options, Icahn Master directly wrote 1,309,342 Put Options, Icahn Master II directly wrote 458,338 Put Options, and Icahn Master III directly wrote 173,085 Put Options.</footnote>
        <footnote id="F8">The Call Options reference an aggregate of 3,813,573 Shares, have an exercise price of $5.65, expire on June 7, 2012, and provide for physical settlement. The Put Options reference an aggregate of 3,813,573 Shares, have an exercise price of $5.65, expire on June 7, 2012, and provide that they settle in cash.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>CARL C. ICAHN</signatureName>
        <signatureDate>2010-06-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
