QuickLinks -- Click here to rapidly navigate through this document

Exhibit 99.1

Certification of CEO and CFO Pursuant to
18 U.S.C. Section 1350,
as Adopted Pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002

        In connection with the Quarterly Report on Form 10-Q of Zenith National Insurance Corp. (the "Company") for the quarterly period ended September 30, 2002 as filed with the Securities and Exchange Commission on the date hereof (the "Report"), Stanley R. Zax, as Chief Executive Officer of the Company, and William J. Owen, as Chief Financial Officer of the Company, each hereby certifies, pursuant to 18 U.S.C. §1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that, to the best of his knowledge:

        (1)  The Report fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934; and

        (2)  The information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

/s/  STANLEY R. ZAX
Name:	Stanley R. Zax
Title:	Chairman of the Board and President
Date:	November 8, 2002
/s/  WILLIAM J. OWEN
Name:	William J. Owen
Title:	Chief Financial Officer
Date:	November 8, 2002

        This certification accompanies the Report pursuant to § 906 of the Sarbanes-Oxley Act of 2002 and shall not, except to the extent required by the Sarbanes-Oxley Act of 2002, be deemed filed by the Company for purposes of §18 of the Securities Exchange Act of 1934, as amended.

QuickLinks

Certification of CEO and CFO Pursuant to 18 U.S.C. Section 1350, as Adopted Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002