 An undrafted player, Mullen was an all-star for the Boston College Eagles before turning professional in the Blues'organization. He was named the Central Hockey League (CHL) rookie of the year in 1980 and most valuable player in 1981 as a member of the Salt Lake Golden Eagles. He won the Lady Byng Memorial Trophy as the NHL's most gentlemanly player on two occasions as a member of the Flames, as well as the NHL Plus-Minus Award. Mullen was named to the First All-Star Team in 1988 – 89 and played in three NHL All-Star Games.
