Before playing in the NHL, Mortson played 1 season with the Tulsa Oilers of the United States Hockey League (USHL). He joined the Toronto Maple Leafs in the 1946–47 NHL season. During his time with the Maple Leafs, he and Jimmy Thomson were known as the "Gold Dust Twins". The two of them helped the Maple Leafs win four Stanley Cups in 1947, 1948, 1949, and 1951.