Johnny played in the National Hockey League from 1930 to 1940. During this time, he played for the New York Americans, Boston Bruins, and Montreal Canadiens. He also played for the Providence Reds of the American Hockey League. He won the Stanley Cup in 1931 with the Montreal Canadiens. Gagnon loved to tell the story of how, as a Canadiens "wanna-be", he filled his pockets with 10 pounds of rocks during a weigh-up and having impressed Canadiens brass with his 150 lbs, got a tryout with the team,(who had formerly shunned him as being "too light for pro hockey"). Gagnon was a modest sort who gave all the credit to his two superstar linemates ( Howie Morenz and Aurel Joliat ) claiming he'd simply pass them the puck, stand back, and get the assists.