As a youth, Lemelin played in the 1965, 1966 and 1967 Quebec International Pee-Wee Hockey Tournaments with a minor ice hockey team from Orsainville, Quebec City.

After playing two seasons in the QMJHL with the Sherbrooke Beavers, Lemelin joined the North American Hockey League’s Philadelphia Firebirds. He was drafted by the Philadelphia Flyers in the seventh round, 125th overall, in the 1974 NHL amateur draft, and was also selected by the Chicago Cougars in the 1974 WHA Amateur Draft; however, he never played in the WHA, due to an argument with management. His NHL debut came with the Atlanta Flames during the 1978–79 season.

Lemelin married his wife, Rona, in July 1976. They have two children together, Brian and Stephanie.

Lemelin remained with the Flames through their move to Calgary, where he enjoyed the most success of his career. He split goaltending duties with Pat Riggin and then Don Edwards, earning the starting job in 1983, In 1984, he was named to the Canadian team that played in the 1984 Canada Cup. He played in two games, earning a win and a loss. After losing his starting job in Calgary to Mike Vernon, he joined the Boston Bruins for the 1987–88 season, and teamed with Andy Moog to win the NHL’s William M. Jennings Trophy (for fewest team goals allowed) in the 1989–90 season.

After battling numerous injuries Lemelin retired during the 1992–93 season with his final game on December 29, 1992 in Winnipeg against the Jets. In his NHL career, he appeared in 507 games and had only two losing seasons.

After 13 seasons as the goaltending coach for the Philadelphia Flyers, he was replaced on June 25, 2009, by Jeff Reese. As of 2010, he was active in Boston area charity hockey games, playing as a goaltender with the Boston Bruins Alumni exhibition team.