He played for the Toronto Maple Leafs, Chicago Black Hawks, and Detroit Red Wings, winning four Stanley Cups with Toronto. He also played in eight NHL All Star games.
In 1946–47 Mortson joined the NHL’s Toronto Maple Leafs, where he played for the next six seasons.
He and fellow defenceman Jim Thomson were known as the “Gold Dust Twins”, and the two helped the Maple Leafs win Stanley Cups in 1947, 1948, 1949, and 1951.
In the 1948 All Star game, Mortson and Gordie Howe squared off in an All-Star Game and, as of 2015, are the only players to fight in an NHL All-Star Game. In 1950, Mortson was named to the league’s first all-star team.
In 1952 Mortson was traded to the Chicago Black Hawks, along with Cal Gardner, Ray Hannigan, and Al Rollins, for Harry Lumley.
Mortson played for the Black Hawks for six seasons. In 1956–57 he led the league in penalty minutes for the fourth time.
He was then traded to the Detroit Red Wings in 1958 and played one season for them.
Mortson played 797 games and had 198 points and 1,380 penalty minutes in his 13-year NHL career. He was known for his physical play and got into numerous fights.