At 36 years old and approaching the end of his career, the 1989 Stanley Cup playoffs was potentially his last chance at winning the Stanley Cup. The Flames defeated the Vancouver Canucks, Los Angeles Kings and Chicago Blackhawks to set up a Stanley Cup Finals rematch of the 1986 Stanley Cup Finals with the Montreal Canadiens. McDonald was left out of the Flames' lineup for the third, fourth and fifth games, but with Calgary leading the series three wins to two, head coach Terry Crisp felt that if the Flames were to win the championship, McDonald deserved to be in the game. Crisp's decision paid dividends, as midway through the sixth game, McDonald stepped onto the ice after serving a penalty to join Håkan Loob and Joe Nieuwendyk in a three-on-one rush toward the Montreal goal. Loob passed the puck up to Nieuwendyk, who saw a streaking McDonald coming up the right side of the ice. McDonald received the pass then shot the puck over Montreal goaltender Patrick Roy's glove to give the Flames a 2–1 lead. Doug Gilmour added two goals, and Calgary won the game 4–2 to earn the franchise's first Stanley Cup championship. League president John Ziegler presented the trophy to McDonald, co-captain Jim Peplinski and alternate Tim Hunter. Wearing the captain's "C" for the game, McDonald was the first member of the Flames to carry the trophy as the team paraded it around the Forum in celebration.