In 1938, aged 14 and weighing 3st 12lb (24.5 kg), Athol Mulley joined Bayley Payten's stable as an apprentice. Throughout a long career Mulley was a colourful and controversial figure on the Australian turf, often at odds with officialdom.

Soon after completing his apprenticeship in 1945, Mulley was thrust into national prominence as the rider of Bernborough. Mulley and Bernborough came together in December, 1945, to win the Villiers Stakes at Randwick. Thereafter, horse and rider put together 15 consecutive wins, a sequence which was only broken when Bernborough ran 5th in the 1946 Caulfield Cup, and Mulley was replaced as his rider. Mulley capped a successful 1945-46 season by winning the Sydney jockeys' premiership.

For the next 20 years Mulley was among Australia's foremost riders. His successes included 2 AJC Derbies, 2 Epsom Handicaps, 3 Golden Slipper Stakes, 3 Newmarket Handicaps, a Metropolitan Handicap, a Brisbane Cup, and most of the country's important weight-for-age events. In 1960-61 he won a second jockeys' premiership.
Mulley retired for the first time in 1969, but returned to the saddle and did not finally quit riding until injury forced him to do so in 1978. Altogether it is believed that he won some 2,000 races, including 110 feature events and more than 40 at Group 1 level.
Mulley was inducted into the Australian Racing Hall of Fame in 2012.