Centre John McCormack first came to hockey prominence in 1943 when he joined the St. Michael's Majors of the OHA. He immediately became one of the team's top offensive producers and finished the year with 18 goals and 48 points in 24 games. In his second and final year of junior hockey, McCormack played in just 15 games for the Majors, but still managed to contribute 18 goals and 41 points. McCormack was one of the team leaders throughout the playoffs and helped propel the Majors to the 1945 Memorial Cup championship. Other notables from that team included; Gus Mortson and Jimmy Thompson with Joe Primeau serving as head coach.
At the age of 20, McCormack signed a deal to play for Tulsa of the USHL. He missed the entire 1946-47 season while taking on full-time seminary studies.
In 1947-48, McCormack got his first opportunity to play in the NHL and dressed for three games with the Toronto Maple Leafs. However, it was not until several years later when McCormack got his first extended role with the Maple Leafs when he skated in 34 games in 1949-50 after signing a free-agent contract.
In September, 1951, McCormack was traded to the Montreal Canadiens for cash. He played a total of 113 games in a Habs' uniform until being claimed by Chicago in the 1954 Intra-League Draft. McCormack played 63 games with the Blackhawks, but was part of a multi-player trade with Detroit in the offseason. However, the Red Wings were strong at centre and not in need of McCormack on the roster, so he spent the entire 1955-56 season in the WHL with the Edmonton, retiring after just one season there at the age of 30.