During the 1944 season, Clapper filled in as interim coach when Art Ross took ill. In 1945 Ross retired as Bruins' coach, retaining his general manager's position, and named Clapper as player-coach, the only one in team history. Clapper retained his team captaincy until his retirement as a player in 1947, ultimately serving as team captain for longer than any NHL player until Ray Bourque surpassed his total in the 1990s.