In mid-1934, Greenhalgh had some amateur portrait drawings published in The Telegraph in Brisbane. His career began doing cute animal drawings for John Sands greeting cards.

Around 1945, he drew 'Aussie Koala', a one-page story for Frank Johnson's one-shot The New Terrific Comics.

In 1948, he was employed as a meat worker in Brisbane while creating with his friends a Disney-inspired animation using Australian animals. He began his animation career working on advertising films in Queensland (Sunday Mail 13/5/1948). In 1952, he joined Eric Porter Studios in Sydney and was art director in 1954.

By 1956, he had his own Sydney-based animation studio, Rowl Greenhalgh Productions. The studio collaborated with International Television Services on Tune Cartoons, a series of three-minute colour animated cartoons set to popular songs, including one based on Waltzing Matilda (1957). The series was shown on ABC television and sold to the United States (beginning with KPIX in San Francisco).

Rowl Greenhalgh Productions also worked on the 1959 animated short Christmas Daze for the ABC. The studio produced commercials in the late fifties until about 1960.

In the sixties, Greenhalgh did animated advertisements for Visatone Television. He became the animation director at Ajax films in the late sixties and worked with Marcia Hatfield on the 26-part animated series, Eddie's Alphabet (featuring Eddie the Earthworm) The series was regularly repeated on ABC television and sold to New Zealand, Hong Kong and Singapore. Greenhalgh illustrated television-related children's books by Marcia Hatfield, Eddie's Alphabet (1969) and Let's Explore with Rosemary (with Rosemary Eather, 1970).

In the seventies, Greenhalgh again worked for Eric Porter as director of the animated television commercials division. He returned to newspaper commercial advertising work before retiring.

He is reported to have been an early Disney artist and a family friend of Walt Disney. This seems unlikely and may have helped him fund his retirement by selling a number of Disney-related paintings.