He began his film career in the early 1930s as an MGM publicist and assistant to David O. Selznick. His first screen credit was "revolutionary sequences arranged by" in David O. Selznick’s 1935 version of A Tale of Two Cities. Lewton also worked as an uncredited writer for Selznick’s Gone with the Wind, including writing the scene where the camera pulls back to reveal hundreds of wounded soldiers at the Atlanta Depot.

In 1942, Lewton was named head of the horror unit at RKO studios. He was paid $250 a week. As head of the B-horror unit he would have to follow three rules: each film had to come in under a $150,000 budget, each film was to run under 75 minutes, and Lewton's supervisors would supply the title for each film.

Lewton's first production was Cat People. The film was directed by Jacques Tourneur, who subsequently also directed I Walked With a Zombie and The Leopard Man for Lewton. Made for $134,000, the film went on to earn nearly $4 million, and was the top moneymaker for RKO that year. This success enabled Lewton to make his next films with relatively little studio interference, allowing him to avoid the sensationalist material suggested by his film titles, instead focusing on ominous suggestion and themes of existential ambivalence.

Lewton always wrote the final draft of his films' screenplays, but avoided an on-screen co-writing credit except in two cases (The Body Snatcher and Bedlam) for which he used the pseudonym "Carlos Keith". (He had previously written a novel, Where the Cobra Sings, under the Carlos Keith pen name.)

After Jacques Tourneur left RKO's horror film unit, Lewton gave first directing opportunities to Robert Wise and Mark Robson.

Lewton died of a heart attack on March 14, 1951, at the age of 46. A new documentary film on Lewton, Val Lewton: The Man in the Shadows, narrated by admirer Martin Scorsese, premiered on Turner Classic Movies on January 14, 2008.