He played senior hockey from 1935 to 1943 for the Brockville Magedomas, Cornwall Flyers, Quebec Aces, Montreal Royals, and the Ottawa Commandos. In 1942-43 he turned pro in the playoffs with the Montreal Canadiens. McMahon played all of the 1943-44 season with Montreal and won the Stanley Cup. In 1944-45 he was back down with the Montreal Royals but he played 13 games with the Canadiens in 1945-46. He also played 2 games for the Boston Bruins as part of a previous trade. In 57 NHL games he scored 7 goals and 18 assists. His son Mike McMahon, Jr. also played in the NHL.