Vernon was born Mario Lippert in Baden, Switzerland, to a Swiss father and an American mother, and was fluent in German, English and French. Originally a stage and radio actor, he worked primarily in France and became a well-known supporting actor after 1945 by playing villainous Nazi officers in French films. Jean-Pierre Melville's Le Silence de la mer, in which he played a gentle anti-Nazi German officer, made him somewhat famous, but, in part due to his looks and Swiss accent, he was subsequently relegated to playing Gangsters and heavies.