Jim Cassidy was well established as a jockey in New Zealand before crossing the Tasman to make Australia his home. He had twice won the Apprentices' Premiership, and in 1981-82 also won the New Zealand Jockeys' Premiership with 120 winners. By the time he moved permanently to Australia he had eight Group 1s to his credit.
Although he had his first Group win in Australia in 1981 in the QTC P.J. O'Shea Stakes, it was his dramatic win in the 1983 Melbourne Cup on Kiwi which put him on the racing map. Offered the opportunity to ride for Brian Mayfield-Smith at Nebo Lodge, Cassidy was fourth on the Sydney jockeys' premiership in 1984-85, his first full season riding in Australia. The following season he won the premiership, the same year Mayfield-Smith broke T.J. Smith's 33 year stranglehold on the trainers' premiership.
Cassidy did not win the premiership again but during the next 15 years he was regularly in the top 10 riders in Sydney in the number of winning rides. As Cassidy often said, he was more interested in the quality of his wins than the quantity, and he concentrated his efforts on the big event meetings. In 1985-86 he rode 28 Group winners, and in 1989-90 and 1992-93 more than 20 in each season. Almost every feature race on the calendar fell to him. In 1997, after one of his several disqualifications, he made a triumphant comeback on the champion Might and Power, winning the Caulfield Cup /Melbourne Cup double, and in 1998 partnering Might and Power to win the W.S. Cox Plate. Added to these, his win on Ha Has in the 2001 Golden Slipper made him one of a handful of jockeys to have won the Grand Slam of Australian racing.
Although his riding opportunities have declined in recent years, he showed he was no spent force when, at the 2009 Sydney autumn carnival, he won the AJC Derby, AJC Doncaster Mile and the STC George Ryder Stakes.
He brought up his 100 Group 1 wins when he took out the Coolmore Stud Stakes at Flemington on AAMI Victoria Derby day 2013.
Cassidy was inducted into the Australian Racing Hall of Fame in 2012.