1960 film, Exodus.

In 1960, Mainwaring was hired by fantasy-film producer-director George Pal to write the screenplay for the MGM Studios film Atlantis, the Lost Continent, released in 1961. He based his script on a play written by Gerald Hargreaves in 1945. Toward the end of his career, in the 1960s, he wrote for TV shows like The Wild Wild West