Simmer split his first three professional seasons between the California Seals (which relocated in 1976 to become the Cleveland Barons) of the NHL and the Salt Lake Golden Eagles of the CHL. He was traded in 1977 to Los Angeles, but spent almost all that season with the Springfield Indians of the AHL, winning all-star accolades. The next season Simmer was promoted halfway through the season to the Kings, and scored a prolific 21 goals in only 39 games. From that point on he was regarded as a star in the NHL.

While with the Kings, he played left wing on the famed “Triple Crown Line” with Marcel Dionne and Dave Taylor (all-time franchise leaders in points and games played, respectively). Simmer had back-to-back 56-goal seasons in the early 1980s, and was also a NHL First Team All-Star in 1980 and 1981.

Simmer was traded at the beginning of the 1984-1985 season to the Boston Bruins, where despite the effects of several injuries throughout his career, he starred for three more seasons. In 1986, Simmer won the Bill Masterton Trophy for his sportsmanship and dedication to hockey.

Simmer played his final NHL season for the Pittsburgh Penguins in 1987-1988. He subsequently played for a season in Europe and parts of two seasons as a player-coach in the minor leagues before retiring. In an interesting side note, Simmer was formerly married to former Playboy Playmate of the Year Terri Welles.