Janson became well known for his role as Jimmy Harker in the 1960s British soap opera, The Newcomers. He later starred in the sitcom Get Some In!. He also replaced Richard Gibson as the Gestapo officer Herr Otto Flick in the BBC sitcom 'Allo 'Allo! in its final 1992 series, after playing Adolf Hitler's double in an episode of series 8.
David was married to Actress Debbie Arnold (who played April Branning in soap opera EastEnders) for 13 years and divorced her in 2002.
