Greenop was born in London and his family migrated to Australia in 1923. Aged 13, his father became ill and Greenop left school. In 1928 he started work with The Daily Telegraph as a copy boy.

In 1937, Kenneth Murray engaged him as the editor for his new Man magazine. He also wrote many articles and fiction stories for Man. He later became Editor in Chief of KG Murray's magazine publications.

Greenop was passionate about history and wrote a number of books on historical subjects. In 1947, while 'Editor in Chief' for Murray, he wrote History of Magazine Publishing in Australia.

During the 1950s he wrote detective novels and stories, including a series of pulp novels featuring detective Max Strong for Cleveland (as Robert Dudgeon). In the 1960s, he published a series of twelve children's books (the Moorooba books), illustrated by Hal English and Col Cameron.