Bill Mahony started swimming at age five and entered his first competition at age eight under the guidance of fellow BC Sports Hall of Famer, Jack Pomfret. Mahony was a member of the Canadian Dolphin Swim Club and was coached by another BC Sports Hall of Fame inductee Deryk Snelling.

Mahony was a bronze medalist at the 1966 British Empire and Commonwealth Games in the 200m breaststroke. Four years later at the 1970 British Empire and Commonwealth Games, he won three gold medals in the 100m and 200m breaststroke and the 400m medley relay.
In his third British Empire and Commonwealth Games in 1974, Mahony helped the Canadian 4x100m medley relay team to a gold medal.

Mahony had great success at the Olympic Games during his career as well. He made the semifinals in the 100m breaststroke at the 1968 Olympic Games in Mexico City and helped the Canadian 4x100m medley relay team to a seventh place finish. Four years later at the 1972 Olympic Games in Munich, Mahony was a bronze medalist in the 4x100m medley relay.

In addition, he helped the Canadian 4x100m medley relay teams to silver medals at the 1967 and 1971 Pan American Games. He was also a six-time Canadian senior record holder in the 100m and 200m breaststroke.