Amid the tumult last March of the Trump administration’s abrupt firing of Preet Bharara from his post as the United States attorney in Manhattan, his deputy, Joon H. Kim, a virtual unknown to the public, quietly stepped in.

It did not take long for Mr. Kim to be thrust into the spotlight.

Within days, he announced that the office would not file charges after what he said was a “thorough investigation” of Mayor Bill de Blasio’s campaign fund-raising. In the ensuing months, Mr. Kim’s office indicted the man who drove a pickup truck down a crowded West Side bike path, killing eight people in the deadliest terrorist attack in New York since Sept. 11, and it filed charges against nearly a dozen people in a college basketball corruption scandal.

And on the day Mr. Kim learned he was being replaced by a prosecutor appointed by Attorney General Jeff Sessions, his office won a high-stakes trial of a Turkish banker in a conspiracy to violate United States sanctions on Iran.

Indeed, Mr. Kim’s 10 months as acting United States attorney were anything but quiet.

Mr. Kim, in a series of recent interviews, reflected on his brief but busy tenure leading the United States attorney's office for the Southern District of New York, a job he took under what he called “unusual and difficult" circumstances.