Former Manhattan District Attorney Robert Morgenthau said he’s joining the New York-based law firm of Wachtell, Lipton, Rosen & Katz.

Morgenthau, 90, left office last month after 34 years as New York City’s top prosecutor. At Wachtell, the firm known for advising Wall Street clients on merger advice, he will be of counsel, a non-partner position.

“I have long admired Wachtell Lipton for not only its outstanding reputation as a law firm, but also for its devotion to and support for advancing the law and the legal profession, education and service to New York and the nation,” Morgenthau said in a statement.

Morgenthau began his prosecutorial career when President John F. Kennedy appointed him U.S. Attorney in 1961. As district attorney he prosecuted money-laundering, securities-fraud and tax-evasion cases, including one against former Tyco International Ltd.’s former chief L. Dennis Kozlowski, who was convicted in 2005 of looting the company.

Wachtell has its sole office in New York. It advised Bank of America Corp. in its purchase of Merrill Lynch & Co., JPMorgan Chase & Co. in its purchase of Bear Stearns Cos. and Wells Fargo & Co. in its acquisition of Wachovia Corp. A co- founder, Martin Lipton, created the “poison-pill” takeover defense.