In the post-war era, Klein drew for a variety of publishers. For DC Comics, nearly ten years before teaming with penciler Curt Swan on various Superman titles, Klein inked him on a "Boy Commandos" story in "World's Finest Comics" #21 (March–April 1946). For American Comics Group (ACG), Klein worked on such horror/suspense titles as "Adventures into the Unknown", "Forbidden Worlds", and "Out of the Night". For Atlas Comics, Marvel's 1950s iteration, Klein penciled but mostly inked stories for such comics as "Marvel Tales", "Sports Action", "Wild Western", and "Space Squadron", for which he drew the backup feature "Blast Revere". By late in the decade he was also doing stories for Prize Comics' "Black Magic."