A late bloomer who struggled during his junior career,[2] Mahoney first represented Australia at the 2012 World Cup winning three medals at the age of 22.

[7] In the final, Mahoney was replaced by Richardson and with D'Orsogna, Jarrod Killey and Robert Hurley won silver in a new Oceanian record time of 6:52.29.

[16] In Singapore, the foursome of Mahoney, Regan Leong, Brittany Elmslie and Emma McKeon won silver in the mixed 4 × 50 metre freestyle relay.

[17] At the penultimate leg in Tokyo in the heats of the mixed 4 × 50 metre freestyle relay D'Orsogna, Leong, the Campbell sisters Bronte and Cate set a new world record time of 1:31.13.

[18][19] In the final, Mahoney replaced Leong as the second leg swimmer and together they won gold and set another world record, dropping to the time to 1:29.61.

[20][21] The final leg in Beijing saw the same foursome win gold in the event again but narrowly missing the world record posting a time of 1:30.52.

In April 2016, at the national championships he finished second in the 400 metres individual medley in a personal best time of 4:14.98 to qualify for his first Olympic Games.

[58][59] An emotional Mahoney stated that his greatest inspiration was his aunt Sally McLean who died in August 2015 after a nine-year battle with cancer at the age of 52.