Louis Blues in August 1979. Prior to his NHL debut, Mullen made a significant impact with the Salt Lake Golden Eagles in the Central Hockey League (CHL), earning CHL Rookie of the Year in 1980 and the league MVP Award in 1981. He made his NHL debut in the 1980 Stanley Cup Playoffs with the Blues and became a regular in the 1981-82 season, becoming the first player in NHL history to score at least 20 goals in both minor leagues and the NHL in the same season. Early in his career, he achieved 40-goal seasons multiple times, including scoring 41 goals in 1983-84, a record for an American-born player at the time.
Mullen was traded to the Calgary Flames in 1986, where he continued his prolific scoring. He earned his first Stanley Cup in 1989, leading the postseason with an impressive 16 goals. During the 1988-89 season, he achieved a personal best of 110 points, tallying 51 goals and 59 assists, and was recognized with the NHL Plus-Minus Award for his +51 rating. His exceptional play led to his inclusion on the NHL First All-Star team in 1989.
Mullen concluded his illustrious 16-season playing career in 1997, having amassed 502 goals and 1,063 points in 1,062 regular-season games. Following his retirement from playing, Mullen transitioned to coaching, serving as an Assistant Coach for the Pittsburgh Penguins from 2000 to 2006, a period which included a stint as interim head coach for their AHL affiliate, the Wilkes-Barre/Scranton Penguins. He subsequently joined the Philadelphia Flyers organization as an Assistant Coach from 2007 to 2017. His profound impact on hockey was recognized with the Lester Patrick Trophy in 1995, induction into the United States Hockey Hall of Fame in 1998, and the Hockey Hall of Fame in 2000.