McCallum was born on September 19, 1933 in Glasgow, Scotland. He is best known for his roles as Illya Kuryakin in the The Man from U.N.C.L.E. television series in the 1960s and as Dr. Donald “Ducky” Mallard in the NCIS television show since 2003. He remained on the show for 20 seasons. He was also in The Great Escape film in 1963.
McCallum voiced Alfred in several Batman works, including the Batman: Gotham Knight anime movie, and the Son of Batman and Batman vs. Robin.