After playing two seasons in the QMJHL with the Sherbrooke Beavers, Lemelin joined the North American Hockey League's Philadelphia Firebirds. His NHL debut came with the Atlanta Flames during the 1978–79 season.
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.
Lemelin retired during the 1992–93 season. In his NHL career, he appeared in 507 games. He is currently entering his 13th season as goaltending coach for the Philadelphia Flyers. He also still plays in Boston area charity hockey games to this day, as a member of the Boston Bruins Alumni exhibition team as their goaltender.