After the First World War, in which Kelen participated as a soldier of the Austro-Hungarian Joint Army, he attended from 1919 the painting school Hans Hofmann in Munich.
In the 1920s and 1930s, Kelen worked with his friend and colleague Alois Derso as a political draftsman for newspapers across Europe, including Illustrated London News. Le Rire, Tribune de Genève and Tempo. He also worked as a book illustrator. For example, in the series "What is not in 'Baedeker'" (Berlin band) - here with Derso - and "Wat riveted in Baedeker state" (Amsterdam band).
On October 13, 1938, both emigrated to New York . There they worked for Ken, Esquire, New York Times, New York Post, Christian Science Monitor and the Washington Post.
From 1948 to 1956 Kelen worked as the first television producer of the United Nations Information Office. After retiring, he published numerous books, including a biography of Dag Hammarskjöld and several children's books.