Cormier made his major league debut on August 15, 1991. He started the St. Louis Cardinals' game against the New York Mets, going six innings, giving up one earned run, and striking out two. He pitched for St. Louis through 1994 and was traded to Boston for the 1995 season. In Boston, Cormier split time as a starter and a reliever. His 1995 ERA was 4.07. Following the 1995 season, Cormier was traded to Montreal as part of the Wil Cordero deal. He spent all of 1996 and one game in 1997 for the Expos. In 1998, he signed as a free agent with the Cleveland Indians and began the year in the minor leagues before shoulder problems ended his season. In 1999, Cormier signed a free agent deal to return to Boston. In two seasons, he made 124 appearances for the Red Sox, all of which came in relief. After the 2000 season, the Philadelphia Phillies signed the reliever as a free agent. In the next six seasons with the Phillies, Cormier had his most successful years. In 2003, he posted in 84.2 innings a career-best ERA of 1.70. In 2004, he made 84 appearances, a career high. On July 31, 2006, Cormier was traded from the Philadelphia Phillies to the Cincinnati Reds for pitching prospect Justin Germano. The Reds, leading the National League wildcard race at the time of the trade, sought bullpen help through the trade. The team failed to make the playoffs, however, finishing the season 8 games back in the wildcard race. Cormier's 2006 season with the Reds included 21 appearances and a 4.50 ERA. On April 28, 2007, Cormier was designated for assignment by the Reds after a poor start to the 2007 campaign. He had 3 IP, a 9.00 ERA, and 1 strikeout in his time with Cincinnati that season. On May 13, 2007, the Atlanta Braves signed Cormier to a minor league contract and assigned him to their AAA-affiliate, the Richmond Braves. He played only briefly for them before deciding to retire. In 2008, he joined the Moncton Mets, a senior team based in Moncton, New Brunswick, in an attempt to make a comeback and join the Canadian Olympic Team participating in the 2008 Olympics in Beijing. Cormier had pitched for Moncton 21 years earlier, prior to his major league career.
Prior to the 2006 season, Cormier played for the Canada national baseball team in the World Baseball Classic. Despite winning two of three games, the team failed to advance beyond the first round. While their record matched Team USA and Team Mexico, they were eliminated in the tie breaker because they allowed the most runs. Cormier appeared in two of the games, pitching 1 innings, giving up 1 hit, and allowing no earned runs.
