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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2007-11-01</periodOfReport>

    <issuer>
        <issuerCik>0001399315</issuerCik>
        <issuerName>NEW M&amp;I CORP</issuerName>
        <issuerTradingSymbol>MI</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001321120</rptOwnerCik>
            <rptOwnerName>Daniels John W Jr</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>770 N. WATER ST.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>MILWAUKEE</rptOwnerCity>
            <rptOwnerState>WI</rptOwnerState>
            <rptOwnerZipCode>53202</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>519</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4176.3553</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>Deferred Compensation Plan</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>31.3753</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2007-11-01</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness></transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>20046</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2015-04-26</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>20046</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>20046</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Immediately exercisable upon the original grant date of 04/26/2005.</footnote>
        <footnote id="F2">This option was acquired pursuant to a series of transactions under an Investment Agreement, dated as of April 3, 2007, among the entity formerly known as Marshall &amp; Ilsley Corporation (?Old Marshall &amp; Ilsley?), certain of its subsidiaries and WPM, L.P. (the ?Investment Agreement?).  Pursuant to the Investment Agreement, on November 1, 2007, Old Marshall &amp; Ilsley merged with one of its subsidiaries to effect the formation of a holding company to hold all of the outstanding stock of Old Marshall &amp; Ilsley (the ?Holding Company Merger?).  Immediately following the Holding Company Merger, shares of Issuer common stock were distributed to the former Old Marshall &amp; Ilsley shareholders.  Each option to purchase Old Marshall &amp; Ilsley common stock was converted to an option to purchase a number of shares of common stock of the Issuer equal to the number of shares subject to the Old Marshall &amp; Ilsley option multiplied by a ratio, the numerator of which was the closing price per share Old Marshall &amp; Ilsley common stock on the date immediately prior to the share distribution (the ?Pre-distribution Stock Price?) and the denominator of which was the average closing price per share of Issuer common stock over the sixth through tenth business days following the date of the share distribution (the ?Post-distribution Stock Price?).  The adjusted exercise price of each converted option is equal to the exercise price per share of Old Marshall &amp; Ilsley common stock subject to such option immediately prior to the share distribution divided by the quotient obtained by dividing the Pre-distribution Stock Price by the Post-distribution Stock Price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Jodi W. Rosenthal, as attorney-in-fact</signatureName>
        <signatureDate>2007-11-14</signatureDate>
    </ownerSignature>
</ownershipDocument>
