McCallum's first movie work was in "Ill Met by Moonlight" (1957), followed "The Secret Place" (1957). Later, he worked in "The Long and the Short and the Tall" (1961), "Billy Budd" (1962), and "Freud" (1962). McCallum played a role in the movie "The Great Escape" (1962), with Steve McQueen and directed by John Sturges. In 1965, he worked in the in biblical movie "The Greatest Story Ever Told" (1965). In this movie he played Judas. In 1966, he co-starred in "Around the World Under the Sea" (1966), with Lloyd Bridges.