Billy Coutu began his ten-year NHL career in 1917-18 with the advent of the new league, playing for the Montreal Canadiens. That year Coutu scored two goals in 19 games. He played another two years in Montreal, patrolling the defense while contributing five goals and six points in 32 games. In 1919, the Stanley Cup finals between the Montreal Canadiens and Seattle was cancelled due to a major outbreak of influenza in Canada and the northern U.S.
In 1920-21, Coutu headed west to join the Hamilton Tigers. In 24 games, he scored eight goals and four assists while picking up 74 minutes in penalties. Despite that successful individual season, Coutu returned to the Montreal Canadiens the following year, playing in 23 games and scoring four goals and seven points.
Coutu remained in Montreal for five seasons during his second tour of duty with the club, scoring a total of 17 goals and 29 points. His final year was in 1926-27, when he played in 41 games for the Boston Bruins, scoring two points.
He continued to play professional hockey for four more years with the New Haven Eagles and the Newark Bulldogs of the CAHL and the Minneapolis Millers of the AHA, retiring in 1931 at the age of 39.