He played for the Toronto Maple Leafs, Montreal Canadiens, Buffalo Sabres, Los Angeles Kings, and New York Rangers.

[1] Duff also briefly served as head coach for the Leafs during the 1979–80 season.

Standing 5 feet 10 inches tall and weighing 163 pounds, he was renowned as one of the top small players of his era.

After two years at St. Michael's (1953–1955), Duff joined the NHL, playing his first game in 1955 in a scoreless match between the Montreal Canadiens and the Toronto Maple Leafs.

He opted to join the NHL full-time for the 1956-57 season, forgoing his final year of junior eligibility.