Mulhern started his National Hockey League career with the Atlanta Flames in 1975, after being picked eighth overall in the 1975 Amateur Draft. He also played for the Toronto Maple Leafs, Los Angeles Kings, and Winnipeg Jets. He retired after the 1981 season, due to declining play, which came from back injuries and surgery.
In his career, he played in 303 NHL games. He scored 27 goals.