After graduating with a starred First Class degree in English, Jennings undertook post-graduate research on the poet Thomas Gray, under the supervision of a predominantly absent I.

After abandoning what looked like being a successful academic career, Jennings undertook a number of jobs including photographer, painter and theatre designer.

He joined the GPO Film Unit, then under John Grierson, in 1934, largely it is thought because Jennings needed the income after the birth of his first daughter, rather than from a strong interest in filmmaking.

In 1938, he edited an issue of the London Bulletin[3] which included a "collection of texts on the Impact of the Machine" and he used this material to prepare a series of talks to miners in the Swansea Valley while making The Silent Village several years later.

This prompted him to add more material and he obtained a contract from Routledge to prepare it for publication as a book; he worked on it fitfully and thought it was almost ready just before his death.

His daughter, Mary-Louise, asked Charles Madge to assist in finally editing it for publication in 1985 as Pandaemonium, 1660–1886: The Coming of the Machine as Seen by Contemporary Observers.

(1940), Words for Battle (1941), A Diary for Timothy (with a narration written by E.M. Forster, 1945), The Dim Little Island (1948) and Family Portrait (his last completed film, which tells of the Festival of Britain, 1950).