Sullivan has also played for the San Jose Sharks, Calgary Flames, Boston Bruins, and Phoenix Coyotes.

He formerly served an assistant coach with the Tampa Bay Lightning, New York Rangers, and Vancouver Canucks, and subsequently in player development with the Chicago Blackhawks for one year.

Sullivan was named head coach of the United States men's national ice hockey team for the 2022 Winter Olympics, but with the NHL withdrawing from the Olympics due to a COVID-19 surge, David Quinn, former head coach of the New York Rangers, was named as his replacement.

He elected to remain at BU to finish school, and in 1990, he began an 11-year National Hockey League career in which he accumulated 54 goals, 82 assists, 136 points and 203 penalty minutes in 709 games.

After the lockout, Sullivan and the Bruins struggled to win in the new NHL, as they ended the 2005–2006 season with a dismal 29–37–16 record, missing the playoffs and finishing last in the Northeast Division.

He was subsequently fired by the incoming general manager Peter Chiarelli on June 27, 2006, and was replaced by Dave Lewis.

[16] On June 12, 2016, Sullivan became just the sixth head coach in NHL history to win the Stanley Cup after being hired mid-season.

[23] On April 12, 2023, with the New York Islanders victory over the Montreal Canadiens, the Penguins missed the playoffs for the first time since the 2005–06 NHL season.