Dunc McCallum played two years of junior hockey with the Brandon Wheat Kings and was loaned to the Edmonton Oil Kings for the 1960 Memorial Cup.
McCallum turned pro in 1960-61 with the Fort Wayne Komets. After stints in Seattle, Sudbury and Vancouver, he got his first opportunity to play in the NHL in 1965-66, suiting up for two games with the New York Rangers who had the likes of Jean Ratelle, Vic Hadfield, Rod Gilbert and Ed Giacomin in their lineup. The team was bolstered by a strong defensive corps that included Harry Howell, Jim Neilson and Rod Seiling, so earning a regular shift was not an easy task.
The advent of expansion in 1967 gave McCallum and many other career minor-leaguers to that point another shot at making the NHL. He joined the Pittsburgh Penguins for 32 games and earned a regular spot on the roster the following year, suiting up for 62 games. His 1969-70 season was cut short due to injuries but he was back in the Penguins' lineup for 77 games in 1970-71 turning in his best year in the league, scoring nine goals and 29 points.
In 1972-73 the rival WHA formed and McCallum inked a deal with the Houston Aeros. Just prior to the start of the 1973-74 season he broke his leg in an exhibition game against the Los Angeles Sharks and missed the entire season. In November 1974 he signed a free agent contract with the Chicago Cougars where he played in 31 games.
McCallum played one final year of pro hockey in 1974-75 with the Long Island Cougars of the NAHL. In 187 NHL contests he recorded 14 goals and 49 points.