The federal prosecutor who supervised the investigation of Michael D. Cohen, President Trump’s longtime personal lawyer, has left the government and will be joining the law firm Davis Polk & Wardwell, the firm said.

The former prosecutor, Tatiana R. Martins, 41, ran the public corruption unit in the United States attorney’s office in Manhattan for a year, having previously served as the deputy chief.

Ms. Martins led the team that retried Sheldon Silver, the powerful former Democratic speaker of the New York State Assembly, who was convicted on corruption charges in May after a previous conviction was overturned last year.

Image Russell Capone is the new chief of the corruption unit.

The new chief of the corruption unit is Russell Capone, 38, who was Ms. Martins’s deputy. He has investigated allegations of graft among police officials and helped to try Norman Seabrook, the former head of the city’s correction officers’ union. After a jury could not reach a verdict in Mr. Seabrook’s trial in November, a mistrial was declared, and he is to be retried this month.