In 1936 Jennings helped with the organisation of the 1936 Surrealist Exhibition in London, in association with Herbert Read and André Breton. It was at about this time that Jennings, along with Charles Madge and Tom Harrisson helped found Mass Observation and co-edited with Madge the text May the Twelfth, a montage of extracts from observer reports of the 1937 coronation of King George VI and Queen Elizabeth for Mass Observation.
Jennings made only one feature length film, the 70-minute Fires Were Started (1943), also known as I Was A Fireman, a wartime propaganda movie detailing the work of the Auxiliary Fire Service, which blurred the lines between fiction and documentary. This film, which uses techniques such as montage, is considered one of the classics of the genre.
He made a number of notable short films, inclusively patriotic in sentiment and very English in their sensibility, such as: Spare Time; Our Country, The Dim Little Island, A Diary for Timothy (with the narration written by E.M. Forster), Words for Battle, London Can Take It!, and Family Portrait (his last film, which tells of the Festival of Britain). Co-directed with Stewart McAllister, Jennings' best remembered short film, made 1942, is Listen to Britain. Excerpts are often seen in other documentaries, especially portions of one of the concerts given by Dame Myra Hess in the National Gallery while its collection was evacuated for safe-keeping.
He died in Poros, Greece in a fall on the cliffs of the Greek island while scouting locations for a future film on post-war healthcare in Europe. He is buried near T.H. White at the Protestant Cemetery in Athens.
After 2001 this situation was partly rectified: firstly by the feature-length documentary by Oscar-winning documentary-maker Kevin Macdonald, Humphrey Jennings: The Man Who Listened to Britain (made by Figment Films in 2002 for British television's Channel 4); and secondly by Kevin Jackson's monumental 450-page biography Humphrey Jennings (Picador, 2004). In 2003 two of his films, Listen to Britain and Spare Time, were included in the Tate Britain retrospective, A Century of Artists' Film in Britain which featured the work of over one hundred filmmakers. As of 2005, nearly all the films of Humphrey Jennings are available on DVDs.