Born in South Porcupine, Ontario (now part of Timmins), McLellan was signed by the Toronto Maple Leafs and brought to Toronto to play for the St. Michael's Buzzers (1945–46) and then the St. Michael's Majors, playing on the team that won the Memorial Cup in 1947.

McLellan turned professional in 1950, and would play most of the next four seasons with the Pittsburgh Hornets, the Maple Leafs' American Hockey League affiliate.

The team represented Canada at the 1959 Ice Hockey World Championships, winning gold.

He spent two years in Tulsa and was then brought back to the NHL in 1969 to succeed the fired Punch Imlach as head coach of the Toronto Maple Leafs.

A few months later, the 51-year-old McLellan died from an apparent heart attack after raking leaves at his home in Agincourt, Toronto.