The Penguins had put together a young and potent scoring machine and Mullen was added to help provide a winning pedigree and a veteran presence. He would only play 39 Points in 47 Games in 1990-91 (his first season with Pittsburgh) but he was a part of the team that brought the Stanley Cup to the Steel City for the first time. Mullen had a healthy second season in Pittsburgh, scoring 87 Points, and would again help the Penguins win the Stanley Cup, Mullen’s third overall.
Mullen played three more years with Pittsburgh, with the first two seeing him score 70 Points. He would sign as a Free Agent with the Boston Bruins in 1996, where he played for one year before coming back to the Pens for one final year in the NHL.The Right Wing would retire with 1,063 Points in 1,062 Games Played. 325 of his Points were with Pittsburgh. Mullen was inducted into the Hockey Hall of Fame in 2000.