[2] Coming out of junior hockey, he was considered one of the top prospects in 1983 and the Toronto Maple Leafs selected him seventh overall in the 1983 NHL Entry Draft.

In trying to force Courtnall to play a tougher brand of hockey, the club saw his offensive production decrease in the 1987–88 season.

After just nine games into the 1988–89 season, on November 7, 1988, the Maple Leafs dealt Courtnall to the Montreal Canadiens for enforcer John Kordic.

In 64 games, Courtnall put up 39 points with the Canadiens, but his fast skating and stickhandling abilities made him a fan favourite.

Courtnall played strongly in the playoffs, where the Canadiens advanced to the 1989 Stanley Cup Finals where they lost to the Calgary Flames.

Courtnall was a participant in both the second and third seasons of Battle of the Blades, the latter as a last-minute replacement for the late Wade Belak, but was eliminated early in the competition on both occasions.