Lehman initially studied piano, but in the early 1960s became interested in photography and cinema. In 1966, after graduating in Film Studies from the Institut National Supérieur des Arts du spectacle, Brussels, he became a film enthusiast and critic, contributing reviews to weekly publications and magazines. He began making films between 1965 and 1983 when working for Club Antonin Artaud, a readjustment day centre for the mentally ill, using cinema as a therapeutic tool with patients. He later founded the film-based organisations Cinélibre, Cinédit, and AJC, the young filmmakers' workshop.
Initially working with amateurs, he has produced 400 films in Super 8, 16 mm or video, as either shorts, features, documentaries, journals or autobiographies, and has shot 300,000 photographs. Lehman's cinematic work has little public recognition but has been shown at festivals and cine clubs.