Roddy was best known for portraying Cornelius and Caesar in the original Planet of the Apes film series, as well as Galen in the spin-off television series. Also, portraying Augustus in the historical drama Cleopatra (1963), he was nominated for the Golden Globe Award for Best Supporting Actor - Motion Picture. Other titles include The Longest Day (1962), The Greatest Story Ever Told (1965), Inside Daisy Clover (1968), That Darn Cat! (1968), Bedknobs and Broomsticks (1971), The Poseidon Adventure (1972), Funny Lady (1975), The Black Hole (1979), Class of 1984 (1982), Fright Night (1985), Overboard (1987), Fright Night Part 2 (1988) and A Bug's Life (1998). He also served in various positions on the Board of Governors for the Academy of Motion Picture Arts and Sciences.
His family moved from England to the United States in 1940 after the outbreak of World War II. McDowall became a naturalized United States citizen on 9 December 1949, and lived in the United States for the rest of his life. He made his first well-known film appearance at the age of 12, playing Huw Morgan in How Green Was My Valley (1941), where he met and became lifelong friends with Maureen O'Hara. The film won the Academy Award for Best Picture, and made him a household name.
He starred in Lassie Come Home (1943), a film that introduced an actress who would become another lifelong friend, Elizabeth Taylor. That same year, he appeared as Ken McLaughlin in My Friend Flicka, and went on to appear in such films as The Keys of the Kingdom (1944) and The White Cliffs of Dover (1944). In 1944, exhibitors voted him the number one "star of tomorrow".