He joined the St. Michael's Majors of the Ontario Hockey Association Jr. league in 1943–44 and played two seasons for them.

He then turned professional and played for the United States Hockey League's Tulsa Oilers in 1945–46, compiling 48 points in 51 games.

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.

[1] In 1952 Mortson was traded to the Chicago Black Hawks, along with Cal Gardner, Ray Hannigan, and Al Rollins, for Harry Lumley.

[1] After retirement from hockey Mortson was involved in the food and beverage business and lived in Oakville, Ontario.