In its first 24 hours, the new Trump administration has reached into critical U.S. attorney’s offices to quickly appoint new interim leaders, signaling a break from past practice that reflects the importance of frontline prosecutors to the administration’s plans.

In three of the most important federal prosecutors’ offices — two in New York City and one in Washington — new acting leaders have been announced. Two of the picks are respected longtime prosecutors; the third is a Trump loyalist who had pushed for the release of Jan. 6 defendants.

The new acting head of the U.S. attorney’s office in Manhattan is Danielle Sassoon, best known for the successful 2023 prosecution of Sam Bankman-Fried for fraud in his cryptocurrency business.

John Durham, a veteran federal prosecutor on Long Island, will lead the U.S. attorney’s office in Brooklyn. Mr. Durham is the son of John H. Durham, the former special counsel who investigated possible government misconduct in previous inquiries into President Trump.

And in Washington, officials announced that Ed Martin, a conservative activist lawyer, would be in charge of that U.S. attorney’s office, which has overseen the 1,500-plus prosecutions related to the Jan. 6, 2021, riot at the Capitol. Mr. Trump granted clemency in all of those cases on Monday — and Mr. Martin, in his first official act, filed a motion to dismiss all of the Jan. 6 cases still pending.

Typically, when a new president is elected, most of the 93 U.S. attorneys from the previous administration resign and, until a replacement is nominated and confirmed by the Senate, their deputies serve in an acting capacity. That changeover period can last months.