Bill (William) Mahony (born September 16, 1949 in New Westminster, British Columbia) is a former breaststroke swimmer from Canada, who's eight years of competing internationally for his native country began in 1966 at the Commonwealth Games in Jamaica, with a bronze in the 220 yd breaststroke.
Mahony swam in two Summer Olympics, 1968 in Mexico City, and later in Munich (1972) where he won the bronze medal in the 4×100 m medley relay, alongside Erik Fish, Bruce Robertson, and Robert Kasting. At the 1970 Commonwealth Games in Edinburgh he was a triple gold medal winner, in the 100 m and 200 m breaststroke, and the 4×100 m medley. In his last swim, at his third consecutive Commonwealth Games (New Zealand,1974), he helped win gold in the medley relay.