He played for the Montreal Junior Royals from 1934 to 1936 and then graduated to the senior Montreal Royals. Escept for the 1938-39 season with the Streatham Redskins in Great Britain, Mahaffy played for the Royals through the 1944-45 season. In 1942-43 he was called up to the Montreal Canadiens where he scored 2 goals and 5 assists in 9 games. In 1943 he was loaned to the New York Rangers as part of the deal that brought Phil Watson to the Habs. He scored 9 goals and 20 assists in 28 games for the Rangers.