For nearly three decades, until his retirement Thursday, federal prosecutor Thomas Zeno handled some of his office’s most high-profile cases.

He prosecuted D.C. Council member and former mayor Marion Barry on tax-evasion charges, oversaw complex health-care fraud trials and helped lead the charge in court proceedings to limit freedom granted to John W. Hinckley Jr., the would-be assassin who has been confined at a mental hospital since being found not guilty by reason of insanity for shooting President Ronald Reagan and three other men in 1981.

At a recent retirement ceremony — jammed with judges, colleagues, federal agents and even defense attorneys — it was clear that Zeno will be missed for reasons other than his courtroom record.

Colleagues said the approachable former prosecutor was a fountain of knowledge in matters ranging from public corruption statutes and health-care law to the often arcane federal guidelines that help determine how long a defendant must serve in prison. In fact, he was widely known as the “sentencing guideline guru.”

“He was one of those guys that everyone goes to for advice,” said D.C. U.S. Attorney Ronald C. Machen. “We are really going to miss him.”

Zeno is returning to his roots in Cincinnatti. (Linda Davidson/THE WASHINGTON POST)

Just last month, a prosecutor called Zeno from court, seeking assistance with the sentencing guidelines as part of a complicated plea deal. “There are few people in the office who are irreplaceable,” said Assistant U.S. Attorney Kate Connelly, who sought Zeno’s help at least once a month in sentencing matters. “He is one of them. He is one of the only people in the office with the knowledge and patience to help” on such complex issues.

Zeno, 59 — a mild-mannered prosecutor with a gray beard who often wore sweaters around the office even in summer — said retiring and joining a private law firm, Squire, Sanders & Dempsey, was a difficult decision. But he said he thought the change would enable him to spend more time with his family.

Zeno grew up in Cincinnati and attended a Jesuit high school and college. While attending Georgetown University’s law school, he worked as a clerk at the U.S. Attorney’s Office for the District of Columbia and became a prosecutor in 1982.

Soon, he was specializing in cases involving defendants found not guilty by reason of insanity; a few years later, he was tapped to be part of the team working on the Hinckley case.

Their job was to challenge requests by Hinckley’s doctors and attorneys, which sought more freedom from St. Elizabeths Hospital for the would-be assassin. Hinckley has been held at St. Elizabeths since his 1982 verdict. Prosecutors won the early court fights. But, in recent years, U.S. District Judge Paul L. Friedman has granted Hinckley more privileges, including unsupervised visits to his mother’s house in Virginia and the opportunity to get a driver’s license.

Although he has often sided with Hinckley’s attorneys over Zeno’s objections, Friedman said he had enormous respect for the former prosecutor, calling him a “lawyer’s lawyer who doesn’t play games.”

“He’s a consummate professional,” Friedman said. “You can trust him, and he always gets to the heart of the matter.”