Simmer was selected 39th overall by the California Golden Seals at the 1974 Amateur Draft. He split his first three pro seasons between the parent club and the Salt Lake Golden Eagles of the Central Hockey League and remained with the franchise when it relocated to Cleveland and was renamed the Barons in 1976.
In August 1977 Simmer attempted to gain a new lease on life when he signed as a free agent with the Los Angeles Kings. Matters seemed to regress in 1978-79 as the big left winger played three games with L.A. and spent the rest of the year with the AHL's Springfield Indians. The following year he was called up to the NHL halfway through the season and began turning heads for the first time as a pro.
Both Simmer and the Kings were in an optimistic frame of mind at the dawn of the 1979-80 season. That year he exceeded all expectations by scoring 56 goals and making 101 points. Suddenly he was a well-known sports figure throughout North America, and part of the newly formed Triple Crown Line. Made up of Simmer, Dionne and Taylor, that was respected by every NHL opponent. That year Simmer also scored at least one goal in each of 13 straight games to become the first player to threaten Punch Broadbent's record of 16 that dated back over five decades. After the season, he was named to the NHL's First All-Star Team.
The next year he duplicated his 56-goal output but suffered a devastating compound fracture of his leg, toward the end of the season during a game at Maple Leaf Gardens. His regular-season excellence still garnered him a spot on the NHL First All-Star Team for the second year running. The Kings finished fourth in the overall standings, but in the absence of their star left winger were upset by the New York Rangers in the first round of the playoffs.
Simmer spent the off-season and the early stages of the 1981-82 schedule recuperating from his injury. In an odd turn of events in 1982-83, Simmer recorded nearly twice as many assists (51) as goals (29). More important, he stayed healthy and played in all 80 regular-season games for Los Angeles. The team missed the playoffs, but Simmer's season continued with Team Canada at that year's World Championship.
The Triple Crown Line enjoyed one more year of greatness in 1983-84. Simmer himself rebounded with 44 goals and 92 points. Five games into the 1984-85 schedule, he was traded to the Boston Bruins, where he scored 33 goals and averaged a point per game on a team that was more defensively oriented.
Despite experiencing injury troubles in 1985-86, Simmer scored 36 goals in only 55 games, and his remarkable comeback was acknowledged when he was presented the Bill Masterton Trophy after the season. During his last NHL season, with the Pittsburgh Penguins in 1987-88, he spent some time on a line with Mario Lemieux and Craig Simpson but had a difficult year adjusting since he was bounced from one line to the next. He played one last year as a pro with the Eintracht Frankfurt franchise in the West German Bundesliga.
Simmer took a year away from hockey but found he was still missing the game. He provided some offense and experience to the San Diego Gulls of the International Hockey League in 1990-91. After playing one game for the squad in 1991-92, Simmer hung up his skates for good. A resilient and talented sniper, Charlie Simmer scored 342 times in 712 regular-season games.