[1] Mahony began his international career at the 1966 Commonwealth Games in Jamaica, with a bronze medal in the 220-yard breaststroke.

[2] 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.

At the 1972 Summer Olympics in Munich, West Germany, he won a bronze medal in the men's 4×100-metre medley relay, together with Canadian teammates Erik Fish, Bruce Robertson and Robert Kasting.

After taking a year off, in his final international event at the 1974 Commonwealth Games in Auckland, New Zealand, he won another gold medal swimming his lifetime fastest Breastroke leg for the Canadian team in the 4×100-metre medley relay, alongside Brian Phillips, Bruce Robertson and Steve Pickell.