Former Sullivan & Cromwell partner John J. O’Brien admitted yesterday to failing to pay more than $2.8 million in personal income taxes on profits he earned at the firm.

Mr. O’Brien, 48, a mergers and acquisitions specialist who left the firm in 2009, pleaded guilty to four misdemeanors before Southern District Magistrate Judge Henry B. Pitman, conceding he failed to pay taxes in 2005 and 2007, and failed to file income tax returns in 2004 and 2006.