The chief judge of U.S.District Court in Manhattan will return to his old law firm at the end of the summer, after spending nearly two decades presiding over some of New York’s highest-profile civil and criminal cases.

The judge, Michael Mukasey, 64, has been on the bench since 1988. Another judge, Kimba Wood, will replace him as chief judge, a distinction that automatically goes to the most senior judge in the district.

Judge Mukasey said he will return to the law firm Patterson Belknap Webb & Tyler LLP, where he will handle commercial litigation and white-collar defense. He had been a partner with the firm in 1987 when President Reagan nominated him to the bench.

The career of a storied trial judge can be a difficult thing to measure.

“His legacy is the manner in which he has administered justice in the cases that have come before him,” the chief judge of the federal district that includes Brooklyn and Queens, Edward Korman, said. “In that regard Michael Mukasey is an outstanding judge.”

When asked what court decision he would most like to be remembered by, Judge Mukasey responded, by telephone, “I’m not in any big hurry to be remembered. And that is for other people to decide.”

The case that first secured Judge Mukasey a lasting reputation was the 1995 trial of 10 militant Muslims who were convicted of a plot to blow up the United Nations and other landmarks around the city. Following the trial — which lasted the better part of a year — Judge Mukasey sentenced the central defendant, the terrorist-sheik Omar Abdel Rahman and another man, El Sayyid Nosair, to life in prison.

His role in that trial led the U.S. Marshals Service to provide an around the clock guard, according to news reports. There are few people in the country with the same level of security as Judge Mukasey. One criminal defense lawyer who has brought cases before Judge Mukasey, Charles Stillman, said he recalls seeing the judge walking on Park Avenue to synagogue on a Saturday morning three years back.

“I made a bee line towards my friend to say hello and suddenly two huge guys — marshals — step up,” Mr. Stillman said. “The sacrifice he made for his country was to give up his privacy and give up his peace. We owe him for that.”

After the September 11, 2001, terrorist attacks, Judge Mukasey issued the first ruling in a case that has been at the center of the national debate over presidential authority — the case involving Jose Padilla, the American citizen the government held for three years as an enemy combatant before bringing criminal charges against him.

The decision that Judge Mukasey handed down for Mr. Padilla had two components, one of which favored the government and the other Mr. Padilla. He ruled that President Bush did have the authority to hold Mr. Padilla as an enemy combatant without charging him for a crime. But he also ruled that the government must allow Mr. Padilla to see his attorneys. When the government came back and argued that allowing Mr. Padilla — who was being held in a military brig in South Carolina — to meet with his attorneys would harm their interrogation of him, Judge Mukasey reaffirmed his original order.

A 2–1 ruling by a panel on the 2nd U.S. Circuit Court of Appeals would later reverse Judge Mukasey’s decision, finding that President Bush did not have the power to detain Mr. Padilla. The U.S. Supreme Court has yet to decide the matter, first ruling that the case should have originated out of South Carolina and later saying the appeal was moot once the government brought criminal charges against him.

When asked to comment on Judge Mukasey’s departure from the bench, one of Mr. Padilla’s lawyers, Donna Newman, said, “I admire him greatly.” She described herself as “another weeping fan.”

Ultimately, Judge Mukasey will witness the outcome of the Padilla case as a member of the public, not as a jurist. The rest of his cases will be reassigned to other judges. He has no trials currently scheduled for his final month.

Judge Mukasey has a reputation as a no-nonsense judge, who has easily garnered the admiration of the lawyers who come before him on his 21st floor courtroom in the Daniel Patrick Moynihan Courthouse, according to the attorney comments in the most recent Almanac of the Federal Judiciary. He became chief judge in 2000. Senator Schumer submitted his name to President Bush as part of a list of jurists he would like to see nominated to the Supreme Court.

Judge Mukasey has a reputation for not involving himse lf in settlement proceedings, as some of his colleagues often do. And Ms. Newman said he had little patience for cross-examinations that did not stay on topic.

“I do feel his return to private practice unfortunately deprives the court system of one of the most outstanding, competent, experienced, tough, and honest judges on the bench,” said one lawyer and friend, Kenneth Bialkin.

Judge Mukasey has also presided over high-profile civil cases, including World Trade Center developer Larry Silverstein’s battle with the insurers of the trade center. In 2004, he dismissed lawsuits against the Italian insurance company, Assicurazioni Generali, that sought to hold the firm accountable for life insurance policies that were held by Holocaust victims.

In 1992, he ruled against Coors Light beer, which was then seeking to stop an aggressive advertisement campaign against it by Anheuser–Busch. And last year he ruled that French Open champion Anastasia Myskina could not sue GQ over the topless photos that came out of a Lady Godiva photo spread she had posed in.

The Southern District of New York, which includes Manhattan, is one of the country’s busiest and most important district courts. Judge Mukasey— who is set to begin receiving his salary-level pension of $165,000 a year— will step down as the district’s chief judge at the end of the day on August 31, he said.

He will begin working at Patterson Belknap in September, the firm’s chairman, Gregory Diskant, said, adding that Judge Mukasey is “not a vacation type of guy.”

Mr. Diskant said Judge Mukasey’s presence at the firm from more than 18 years ago is still well remembered. The memoranda or “new matter sheets” that he wrote for each new case became famous for the hilarious descriptions that Mr. Mukasey inserted, Mr. Diskant recalled. Back then Mr. Mukasey often defended the New York Daily News against libel suits.

“One of my partners just described the great pleasure he felt that Michael was coming back to the firm because he couldn’t wait to read his ‘new matter’ sheets,” Mr. Diskant said.

Born in the Bronx, Judge Mukasey went to work as a federal prosecutor in Manhattan during the 1970s, rising to become chief of the official corruption unit. He befriended Rudolph Giuliani there and is the subject of a passage in Mr. Giuliani’s book describing how the two would prepare for trial together, with Mr. Mukasey playing the role of a witness while Mr. Giuliani practiced his cross-examinations. As a federal judge, Mr. Mukasey swore in Mayor-elect Giuliani in 1994 and 1998.

Many of New York’s federal judges often stay on the bench until they are too old to continue working. Judge Mukasey said he is leaving his job because if “I’m going to do anything else at this point, now is the time to do it.”

He added, alluding to the far better pay of private practice: “Also my family has been very supportive of my career, and it’s about time for me to return the favor.”

The judge’s move was earlier reported on the law blog of WSJ.com.