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

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2010-11-10</periodOfReport>

    <notSubjectToSection16>1</notSubjectToSection16>

    <issuer>
        <issuerCik>0001022345</issuerCik>
        <issuerName>MARINER ENERGY INC</issuerName>
        <issuerTradingSymbol>ME</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001475641</rptOwnerCik>
            <rptOwnerName>Sherman R. Cris</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>ONE BRIARLAKE PLAZA, SUITE 2000</rptOwnerStreet1>
            <rptOwnerStreet2>2000 WEST SAM HOUSTON PARKWAY SOUTH</rptOwnerStreet2>
            <rptOwnerCity>HOUSTON</rptOwnerCity>
            <rptOwnerState>TX</rptOwnerState>
            <rptOwnerZipCode>77042</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>VP &amp; Chief Accounting Officer</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-11-10</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>4430</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>22599</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2010-11-10</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>D</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>22599</value>
                </transactionShares>
                <transactionPricePerShare>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">On November 10, 2010, 16,756 shares of the issuer's common stock granted to the reporting person under the issuer's Stock Incentive Plan, as amended and restated from time to time (the &quot;Plan&quot;), vested. To satisfy withholding tax obligations in connection with the vesting, the issuer withheld the shares this Form reports were disposed. In accordance with the Plan, the number of shares withheld was determined by reference to the closing price per share of the issuer's common stock on the New York Stock Exchange on November 9, 2010 of $26.50.</footnote>
        <footnote id="F2">Disposed of pursuant to an Agreement and Plan of Merger between Apache Corporation (&quot;Apache&quot;), Apache Deepwater LLC and Mariner Energy, Inc., dated  4/14/2010, as amended (the &quot;Merger Agreement&quot;), on 11/10/2010, the effective date of the merger (the &quot;Effective Date&quot;).  Pursuant to the Merger Agreement, Mariner stockholders could elect to receive the following consideration:  (i) all stock consideration of 0.24347 share of Apache common stock for each share of Mariner common stock, subject to possible proration, (ii) all cash consideration of $26.00 in cash for each share of Mariner common stock, subject to possible proration, and (iii) mixed consideration of $7.80 in cash and 0.17043 share of Apache common stock for each share of Mariner common stock (the &quot;Mixed Consideration&quot;).  If no election was made, the stockholder will receive the Mixed Consideration for Mariner common stock.  Apache announced it expects to announce final results of merger consideration elections on 11/12/2010.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>R. Cris Sherman by Scott D. Josey pursuant to Power of Attorney dated October 29, 2009 (previously filed as Exhibit 24 to Mr. Sherman's Form 3 filed on October 29, 2009)</signatureName>
        <signatureDate>2010-11-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
