[1] Brennan's other television credits include Special Squad, The Flying Doctors, All Together Now, King & Maxwell, State Coroner, Stingers, Flipper, CSI: Miami, and Summerland.

Beginning as a journalist for the Australian Broadcasting Corporation (ABC), by 1981, he had abandoned journalism for television scriptwriting.

From the mid-1990s he worked on American cable shows shot in Australia – of which Flipper is the only named example in the IMDb.

[3] In July 2016, it was announced that Brennan was stepping down as showrunner of NCIS: Los Angeles and that he was handing the reins over to R. Scott Gemmill.

[4][5] Brennan was elected president of the Australian Writers' Guild in January 2019.