[3] Janson made his cinematic debut in 1959 in The Buddenbrooks,[2] and appeared in Helmut Käutner's film Das Glas Wasser (A Glass of Water) in 1960, alongside Gustaf Gründgens and Liselotte Pulver.

[3] He also appeared in films such as Ruf der Wildgänse (The Cry of the Wild Geese, 1961), Das Riesenrad (1961), and Robert Siodmak's Escape from East Berlin (1962), opposite Don Murray, Christine Kaufmann and Werner Klemperer.

[9] He appeared in many other feature films, including spaghetti westerns with Franco Nero, Eli Wallach and other international productions with Tony Curtis, Anthony Perkins, Charles Bronson, and Robert Wagner.

[3][2] Janson appeared in numerous other works for both film and television, including more than 200 episodes of Sesamstraße (the German version of Sesame Street (1980–1985)),[12][7][11] and Zwei Schlitzohren in Antalya (1991–1994).

[13] Janson also appeared in many German, French, Spanish, Italian, and English-language TV films including Der Bunker (1992), Il segno della scimmia (The Sign of the Ape, 1997),[14] and Freundinnen fűr Immer (2003).