He studied at Leighton Park School in Reading and trained at the Royal Academy of Dramatic Art (RADA) in London, where he won the John Gielgud Scholarship.

Allen played David Hunter, who was a shareholder of the Crossroads Motel with Meg Mortimer, Tish Hope and Bernard Booth.

Allen reprised the role in the sequel Five Go Mad on Mescalin (1983), and also appeared in South Atlantic Raiders Part 2 (1990), The Strike (1988) and Oxford (1990), in addition to the feature film The Supergrass (1985).

Allen other screen appearances included roles in television's The Adventures of Robin Hood (1957), Danger Man (1960, 1961), The Avengers (1964), and Bergerac (1990).

[4] In the 2023 ITVX miniseries Nolly, which dramatised the life of his former Crossroads colleague Noele Gordon, Allen was portrayed by Richard Lintern.