
<PAGE>

                                  EXHIBIT 5.1

                   [NORTHROP GRUMMAN CORPORATION LETTERHEAD]

                                  May 30, 2001

Northrop Grumman Corporation
1840 Century Park East
Los Angeles, California  90067

Re:  Registration Statement on Form S-8

Ladies and Gentlemen:

     I am a member of the bars of the States of New York and California.  I am
also Corporation Vice President, Secretary and Associate General Counsel of
Northrop Grumman Corporation, a Delaware corporation (the "Company").  I am
familiar with the Registration Statement on Form S-8 (the "Registration
Statement") being filed by the Company with the Securities and Exchange
Commission under the Securities Act of 1933, as amended, in connection with the
registration of 1,155,665 shares of common stock, par value $1.00 per share of
the Company (the "Common Stock"), issuable under the 1984 Long-Term Stock
Incentive Plan of Litton Industries, Inc. (the "Plan").  In connection
therewith, I have examined the Company's certificate of incorporation and bylaws
and originals or copies certified or otherwise identified to my satisfaction of
such other documents, corporate records, certificates of public officials and
other instruments as I have deemed necessary or advisable for the purpose of
rendering this opinion.

     Based upon the foregoing, I am of the opinion that the shares of Common
Stock issuable under the Plan, when issued, delivered and paid for in accordance
with the Plan and in the manner described in the Registration Statement, will be
validly issued, fully paid and nonassessable.

     I hereby consent to the filing of this opinion as an exhibit to the
Registration Statement.

                               Sincerely,

                              /s/ John H. Mullan
                              --------------------------------------------
                              John H. Mullan
                              Corporate Vice President, Secretary and
                              Associate General Counsel
