Outsider Tapped in Michael Flynn Case is a Former Mob Prosecutor

As a prosecutor, Mr. Gleeson had a reputation for being aggressive — perhaps, some said, overly so — and for taking a humorless, even cold, approach to lawyering. With his tight-lipped manner and wire-rimmed glasses, he was known around the office by a goody-two-shoes nickname: Clark Kent.

As a judge, however, his vision of the criminal justice system, and his sense of empathy, seemed to broaden.

“He began to see many of the inequities that people face — especially the poor and minorities,” Mr. Mehler said. “He became more liberal on criminal matters and a kind of champion for the down and out.”

Mr. Gleeson was an early advocate of federal sentencing overhauls and was a driving force behind bringing special drug courts to Brooklyn, working with an agency known as Pretrial Services to start a program that allowed drug-addicted defendants to avoid prison time by achieving sobriety. In more than two decades as a judge, he regularly visited prisons to ask inmates about the experience of being in custody.

In the financial sphere, Mr. Gleeson oversaw the government’s decision to defer the prosecution of the banking giant HSBC, which was accused of an array of money-laundering violations. While critics attacked the deal, which allowed HSBC to avoid criminal charges, as a slap on the wrist, Mr. Gleeson monitored it regularly to ensure the bank’s compliance, warning prosecutors that such agreements were still subject to judicial oversight.

When Mr. Gleeson left the bench in 2016 and went into private practice, he continued his work on sentencing changes in between more white-collar cases. In the past few years, he has helped free inmates whose prison terms were found to be egregious.

He may have caught Judge Sullivan’s eye this week when he co-wrote an opinion article for The Washington Post, noting that the judge could reject the Justice Department’s motion to dismiss the charges against Mr. Flynn if he wanted. The headline said it all: “The Flynn Case Isn’t Over Until the Judge Says It’s Over.”