Cullen was a cartoonist, actor and scriptwriter. He began as a press artist for Australian Consolidated Press (ACP) working at the Telegraph for two years. Cullen also later worked as a press artist on the Sydney Morning Herald.

In the early fifties, Cullen was providing cartoons for the Bulletin.

During the mid-fifties, he wrote and drew Oscar Comic ("The World's Funniest Detective") for Young's Merchandising. He later created the one-shot The Adventures of the Jolly Swagman (Westbury Promotion, 1974) for distribution via Ampol service stations. He also drew comics for the Kings Cross Whisper during the sixties and 'Fred the Fisherman' for Fishing News in the seventies. 

He moved into TV and was art director at ATN (channel 7) for two years, at Crawford Productions as a set designer for three years, then a documentary writer and director including a TV series on dental health. He is most known for his script writing work on Homicide (1973), The Outsiders (1976) and The Man from Snowy River (1982).

In the late seventies, he also made commercials, including Polytex Polyfilla and Berger Paints.