[4] Cassidy was born in Wangaratta, Victoria, on 4 March 1950, and grew up in the Victorian town of Chiltern, attending Rutherglen High School.

Starting his career as a cadet on the Albury Border Morning Mail in 1969,[1] he moved to the Shepparton News about a year later before being hired as a court reporter for the Melbourne Herald.

[1] Cassidy moved to Washington, D.C., in 1991, to be with his girlfriend, Heather Ewart, who had been posted there as the North America correspondent for the Australian Broadcasting Commission.

[5] Cassidy worked as a correspondent for The Australian before returning to Australia to host the Last Shout and Meet the Press programs on Network Ten.

[6] He returned to the ABC to replace Paul Lyneham as host on The 7.30 Report,[7] before he and his wife, Heather Ewart, were sent to Brussels as European correspondents, sharing the job.

[8] In 2010, Cassidy wrote The Party Thieves: The Real Story of the 2010 Election (Melbourne University Press, October 2010, ISBN 978-0-522-85780-1), which one reviewer called "the standard text on precisely what happened in 2010".

He formerly hosted the sports panel show Offsiders, but he stepped down from this role to write The Party Thieves, and at the end of the 2013 season left the program entirely.