Painter and caricaturist Joseph Otto Flatter was born into a middle-class Jewish family in Vienna, Austria-Hungary (now Austria) in 1894. His brother Richard Flatter (also an émigré to Britain) was a renowned Shakespeare scholar. Flatter’s studies at the Academy of Fine Arts, Vienna, were interrupted by the First World War, in which he served in the Austro-Hungarian Army in South Tyrol, Italy. Afterwards, he worked as a portrait painter in Eastern Europe and as a lecturer in Brno, Czechoslovakia (now Czech Republic). In 1934 he immigrated to Britain, together with his pianist-composer wife Hilde Loewe (Hilda Löwi), settling in St John’s Wood. Flatter’s commissions included the Austrian ambassador, Sir George Frankenstein; the first Director of the London School of Oriental Studies, Sir Denison Ross; and the actor Emlyn Williams, although he is best known for his anti-Hitler political cartoons, begun in 1938, satirising selected quotations from ‘Mein Kampf’. The series was adopted by war charities to form modest touring exhibitions in London, Oxford, Birmingham, and Cambridge.

In 1940, following internment as a so-called 'enemy alien' in Hutchinson Camp, Douglas, Isle of Man, Flatter worked intermittently as a cartoonist with the Ministry of Information (1940–45), also publishing cartoons in ‘Free Austria’, ‘Die Zeitung’ and British and American newspapers including the ‘Daily Telegraph’. He was one of only two foreign artists (alongside Josef Bato) granted a drawing permit to make sketches of the Blitz and its aftermath. Flatter also worked for the Belgian government in exile and for pro-de Gaulle French publications and served in the Home Guard. In 1944, he held a solo exhibition at the Austrian Centre. In 1946 he attended the Nuremberg War Trials with cartoonist David Low as an official war artist. Flatter was naturalised British in 1947. In his later years he focused on picture restoration and researched the work of Old Master painters. In 1976 he presented around 200 pen and ink drawings to the Imperial War Museum, London.

Joseph Otto Flatter died in London, England in 1988. His cartoons were included in Ben Uri's touring exhibition, ‘Forced Journeys: Artists in Exile in Great Britain c. 1933–45’ (Sayle Gallery Douglas, Isle of Man, and Williamson Gallery, Birkenhead, 2010).