He played in the National Hockey League with five teams between 1983 and 1992, winning the Stanley Cup in 1989 with the Calgary Flames.

He joined Washington front office in 2000 and spent 13 seasons in various roles before becoming general manager in 2014, and in 2018 won the Stanley Cup with the team.

After playing his college hockey at Bowling Green State University, MacLellan signed as a free agent with the Los Angeles Kings in 1982.

He would be traded again, this time to the Minnesota North Stars prior to the 1986–87 NHL season, where he would score a career-high 32 goals.

Following the 2023–24 season, MacLellan was succeeded as general manager of the Capitals by Chris Patrick, while retaining his role as president of hockey operations.