Rheal Cormier, a 16-year major-league pitcher who became a U.S. citizen during his time as a durable reliever in the Phillies’ bullpen, died Monday at 53 from cancer.

Mr. Cormier, a native of Canada, spent six seasons with the Phillies (2001-06) and his 363 appearances trail only Tug McGraw as the most in franchise history by a left-handed reliever.

He retired in 2007 with a 4.03 ERA over 683 games with the Cardinals, Red Sox, Expos, Phillies, and Reds.

“Rheal was one of the most vibrant people I’ve had the pleasure of knowing,” Jim Thome said. “He loved baseball, but he always put his family first. Frenchy was the kind of guy who would do anything for you and I’m lucky to have called him my friend for many years. Our time spent together in Philadelphia as teammates was unforgettable. He will be greatly missed but never forgotten.”

Mr. Cormier’s best season came in 2003 when he went 8-0 with a 1.70 ERA, which was the second-lowest mark that season by a reliever and the sixth-lowest by a Phillies reliever since 1900.

He started the season by allowing five runs in two innings in his first appearance, was booed a day later at the final home opener at Veterans Stadium, and responded by allowing five runs over his next 51 games. Pitching coach Joe Kerrigan called him “the secret MVP of our pitching staff.” Manager Larry Bowa called him “resilient.”

“It wasn’t a great feeling,” a humble Mr. Cormier said near the end of that season about the boos. “... I’m just trying to get outs.”

The next season, Mr. Cormier set a franchise record with 84 appearances by a lefthanded pitcher and the most by any pitcher in the last 33 years. The Phillies said Mr. Cormier was “known off the diamond for his kindness and philanthropy” and made generous contributions to the team’s charity.

A Canadian Baseball Hall of Famer, he pitched for Canada in the 2008 Olympics and in the 2006 World Baseball Classic. He became an American citizen in 2004 after taking the citizenship test while the Phillies were in Miami.

“Everything I have comes from this country,” Mr. Cormier said in June 2004. “I’m proud of America. I’ve lived here a long time. I sort of consider myself a U.S. citizen without the paper. I just want to make it legal.”