He played in the National Hockey League with the Detroit Red Wings, Buffalo Sabres, and Toronto Maple Leafs between 1977 and 1984.

McCourt played major junior in the Ontario Hockey Association (OHA, renamed OMJHL during his tenure, today's OHL).

In 1976–77, McCourt led the relocated St. Catharines Fincups as the team won the OMJHL Regular Season Championship.

Before the start of the 1978–79 season, Red Wings general manager Ted Lindsay signed goaltender Rogatien Vachon of the Los Angeles Kings, who was a restricted free agent at that time.

The matter was eventually resolved and McCourt remained in Detroit, but he felt betrayed by the fact that his own NHLPA, led by executive director Alan Eagleson, did not back him against the owners during the lawsuit.

Armstrong won the Red Tilson Trophy as the OHA's Most Outstanding Player in both 1947–48 and 1949–50, the same award that McCourt received in 1976–77.