A undated family photo of Tom Quinn. (Family Photo)

Tom Quinn, a champion boxer at Georgetown University in the 1950s who later worked in investments before becoming a busy character actor in theater, television and film, died Jan. 5 at a hospital in Teaneck, N.J. He was 79.

He had complications from diabetes, his son T.J. Quinn said.

Mr. Quinn was the last boxer at Georgetown to win an intercollegiate title and to be named to the university’s athletic hall of fame. He joined the boxing team after participating in intramural competitions and quickly became one of the top college heavyweights on the East Coast.

He later boxed in the Marine Corps and stayed close to the sport while working as a stockbroker, political consultant, investment adviser and benefits director for the NFL Players Association. For years, he taught boxing — “which I sometimes refer to euphemistically as Advanced Irish Pilates,” he said in 2008 — at local gyms and to students at Georgetown.

But in the last 25 years of his life, Mr. Quinn took on yet another career as a character actor. He never became a star, but with his real-world experiences, his boxing-ring savvy and a round, cherubic face that was like a living map of Ireland, he seldom lacked for work.

Based in Washington since 1975, Mr. Quinn had small roles in “The Pelican Brief,” a 1993 legal drama starring Julia Roberts and Denzel Washington, and in the 1998 spy thriller “Enemy of the State,” with Will Smith and Gene Hackman. He appeared on the TV political drama “The West Wing” and on the gritty detective show “Homicide.” He had a recurring role as a washed-up, cynical police officer in the first season of “The Wire,” the well-regarded HBO series about the drug underworld of Baltimore.

He portrayed a baseball manager in the 1994 movie comedy “Major League II” and was cast as a trainer in the 2007 boxing film “The Hammer.” Mr. Quinn was versatile enough to appear in Shakespearean plays at the Folger and Shakespeare theaters, but he may have had his most challenging — and most fitting — part when he played an aging boxing champion-turned-promoter in the 2000 revival of “The Great White Hope” at Washington’s Arena Stage.

“Acting is the most gratifying thing I’ve done since my boxing days,” he told The Washington Post at the time.

Thomas Michael Quinn was born April 28, 1934, in Queens, N.Y. He first put on the gloves when he was 5, but he also had a youthful flair for acting.

“I always got the lead in the school play,” he told The Post in 2000. “The nuns predicted I would be an actor. But then I hit my teens and had more . . . testosteronical imperatives.”

After enrolling at Georgetown, he began to box in intramural meets, where he was discovered by Marty Gallagher, a onetime prizefighter who coached the university’s boxing team.

Mr. Quinn, a 189-pound heavyweight, was named captain as a senior in 1955, the final year Georgetown had a collegiate boxing team. The National Collegiate Athletic Association stopped sanctioning the sport when a boxer from Wisconsin died after a national championship bout in 1960.

At the Eastern Intercollegiate Boxing Tournament in 1955, held in College Park, Md., Mr. Quinn “was the sensation of the semi-finals,” The Post reported, when he “smashed out an upset decision over the class of the tourney, Nick Georgiade of Syracuse.”

In the tournament’s championship bout with Don Shannon, a football player at the U.S. Military Academy, Mr. Quinn “brought the crowd to its feet screaming encouragement.” He floored Shannon in the first round, shrugged off a bloody nose, then knocked out the Army fighter with a double left hook in the second round.

Mr. Quinn was later named to Georgetown’s athletic hall of fame, and his old boxing gloves are still on campus in a trophy case at McDonough Arena.

After his graduation in 1955, Mr. Quinn served as an officer in the Marine Corps and continued to box until an injury forced him to give up the sport.

In the early 1960s, he moved to New York, where he worked as a stockbroker. He became part of a literary circle that included writers Norman Mailer, George Plimpton and Pete Hamill. Together, they and other investors sponsored a professional boxer, Joe Shaw. With Mr. Quinn his manager in the late 1960s, Shaw became a top-ranked welterweight.

Mr. Quinn briefly worked as a fundraiser for U.S. Sen. John Culver (D-Iowa) before moving to Washington in 1975 as a consultant to the Democratic National Committee. He was director of benefits for the NFL Players Association from 1978 to 1982, then held executive positions through the 1990s with the investment management firms Legg Mason, Provident Capital Management and Morgan Stanley.

His marriage to Marsha Lehman ended in divorce. Survivors include three children, T.J. Quinn, an investigative reporter for ESPN, of Teaneck, Katherine Quinn of McLean and Sarah Quinn of Seattle; three sisters; a brother; and five grandchildren.

In the 1980s, Mr. Quinn decided to take a course in theater at Georgetown, which rekindled his old interest in acting. He appeared in plays at Washington’s Scena Theatre and began to get parts calling for a burly, middle-aged actor.

For the past two decades, he divided his time between the art of drama and the art of self-defense. He coached the club boxing team at Georgetown and taught boxing as a form of fitness and self-protection to Georgetown students, both men and women.

“Boxing is the truest of sporting contests,” Mr. Quinn said in 2008. “It is the sport that every other sport aspires to be — two people trying to find out which is better and also learning something about themselves.”

He added: “If boxing was easy, they’d call it football.”