 Despite being of Canadian nationality he won the ASA National British Championships 220 yards breaststroke title in 1969.At the 1970 Commonwealth Games in Edinburgh, he won three gold medals in the 100-metre and 200-metre breaststroke, and the 4×100-metre medley relay.In Cali, Colombia for the 1971 Pan American Games, Mahony earned a silver medal as a member of Canada's 4×100-metre medley relay team.