Montagu joined the Fabian Society in his youth, then the British Socialist Party, and then the Communist Party of Great Britain. This brought him into contact with the brilliant Russian film makers who were transforming the language of editing and montage in the 1920s. In 1930 he accompanied his friend Sergei Eisenstein to New York and Hollywood; later in the decade Montagu made a number of compilation films, including "Defence of Madrid" (1936) and "Peace and Plenty" (1939) about the Spanish Civil War. He directed the documentary "Wings Over Everest" (1934) with Geoffrey Barkas. As a political figure and for a time a communist, much of his work at the time was on low budget, independent political films. By World War II, however, he made a film for the Ministry of Information. After the war Montagu worked as a film critic and reviewer.