Marie Greyhounds of the Ontario Hockey Association that saw him score 99 points in his only season (1973–74), Simmer was selected in the third round of the 1974 NHL amateur draft, 39th overall, by the California Golden Seals, where he joined a young field of players such as Larry Patey, Rick Hampton, George Pesut, Al McAdam and Stan Weir.

He split his first three professional seasons between the Seals and the Salt Lake Golden Eagles of the CHL, having success in the minors but receiving limited playing time in the NHL.

He was traded in 1977 to the Los Angeles Kings, but spent the season with the Springfield Indians of the AHL, winning All-Star accolades.

Despite injuries costing him significant playing time, Simmer had back-to-back 56-goal seasons and was further named an NHL first team All-Star in 1980 and 1981.

Simmer spent years as a color commentator for the Phoenix Coyotes and the Calgary Flames on Sportsnet.