Montagu was born into enormous wealth, as the third son of Gladys Helen Rachel Goldsmid and Louis Montagu, 2nd Baron Swaythling, a Jewish banking dynasty with a mansion in Kensington. He attended Westminster School and King's College, Cambridge, where he contributed to Granta. He became involved in zoological research.
With Sidney Bernstein he established the London Film Society in 1925, the first British film association devoted to showing art films and independent films. Montagu became the first film critic of The Observer and the New Statesman. He did the post-production work on Alfred Hitchcock's The Lodger in 1926 and was hired by Gaumont-British in the 1930s, working as a producer on a number of the Hitchcock thrillers. His 1928 silent slapstick movie 'Bluebottles' (slang for police) is included in the British Film Institute's History of the Avant-Garde – Britain in the Twenties. The story was by H. G. Wells, and the stars of the film were Charles Laughton and Elsa Lanchester, while the remaining cast were his friends including Norman Haire (also Montagu's doctor), Sergei Nolbandov and Joe Beckett.
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.
In 1933, Montagu was a founder member of the Association of Cinematograph and Television Technicians, holding various positions in the union until the 1960s. He also held post on the World Council of Peace. In 1934 he was founder of the Progressive Film Institute.
Montagu had a keen interest in wildlife conservation, and was a council member of the Fauna Preservation Society for several years. He was friends with the eminent Soviet conservationist and zoologist Prof. A. Bannikov. He had contacts in Mongolia, and was a champion for the conservation of the endangered Przewalski's horse.
Montagu was a champion table tennis player, representing Britain in matches all over the world. He also helped to establish and finance the first world championships in London in 1926.
Montagu founded the International Table Tennis Federation that same year, and was president of the group for more than forty years, not retiring until 1967. He also founded the English Table Tennis Association (ETTA), and served several terms as chairman and president.
Montagu was inducted into the International Table Tennis Foundation Hall of Fame in 1995.
His eldest brother Ewen Montagu, a strait-laced barrister in civilian life, became a Naval Intelligence Officer RNVR in MI6, during the Second World War, privy to the secrets of top-secret Ultra and the mastermind of the successful deception that launched the invasion of Sicily, Operation Mincemeat. He later wrote a best-selling account of that adventure, The Man Who Never Was. Ivor Montagu himself turned out to be working, albeit briefly, for the other side. A 25 July 1940 cable from Simon Davidovitch Kremer, Secretary to the Soviet Military Attaché in London, identified him as the somewhat reluctant new recruit who was supposed to create an "X Group" of like-minded friends. By the following year, Hitler had invaded Russia and the Soviet Union became an ally of Britain's, so that by June, 1941, both brothers were technically working for the same side.
Ivor knew of his elder brother's spy work, but it seems doubtful his brother knew of his. Counter-espionage agents at MI5, however, even Ewen's boss John C. Masterman, seem to have suspected Ivor in general because of his outspoken Communist politics, his hanging around with scruffy Russians and housing a Jewish refugee in his house in the country. By far the greatest suspicions were aroused by Ivor's passionate support of international ping-pong, which seemed so eccentric to MI5 that they assumed it had to be a cover for something else. They even tapped his phone and opened his mail, creating three volumes on him by early 1942, but found nothing specific, much to Ewen's relief, since he was always worried that his own career in MI6 would be adversely affected by the activities of his left-wing brother.
Only after the decryption in the 1960s of Venona telegraphs from March 1940 through April 1942 was he purportedly identified as "Ivor Montagu, the well known local communist, journalist and lecturer," code name "Intelligentsia", in communications from the Soviet GRU. It should, however, be noted that the Verona decrypts, which were finally declassified in 1995, are a highly contested and confusing archive of information, with a welter of code-names, which were frequently changed and not explicitly identified with their real names; hence these decrypts were never used in any U.S. or British trials for treason, despite their availability to prosecutors from the late 1940s.
If "Intelligentsia" was, indeed, Ivor Montagu, his rather haphazard informing had little of the impact of those much more famous Cambridge-educated spies, recruited as idealistic anti-Fascists in the 1930s, who continued to work at the height of the Cold War - Kim Philby, Guy Burgess and Donald MacLean. In 1952, MI5 intercepted a telegram from Ivor Montagu telling Charlie Chaplin how sorry he was to miss him in London when the star visited England that year; the British agency had agreed to spy on Charlie Chaplin for the FBI, who were looking for ways to keep him out of America at the height of the McCarthy Blacklist. Apart from that, they left Montagu and his ping pong tournaments alone.
Montagu was awarded the prestigious Lenin Peace Prize in 1959, given by the Soviet government to a number of recipients whose work furthers the cause of Socialism, primarily outside of the USSR.
Montagu wrote many pamphlets and books, such as Film World (1964), With Eisenstein in Hollywood (1968), and The Youngest Son (1970). He wrote two books about table tennis: Table Tennis Today (1924) and Table Tennis (1936).