Best known by the public for his starring roles as secret agent Illya Kuryakin on The Man from U.N.C.L.E. (1964) and as head medical examiner Dr. Donald "Ducky" Mallard on NCIS (2003).
Along with Leonard Nimoy, Cliff Robertson, Barbara Rush and Peter Breck, he is one of only five actors to appear in both The Outer Limits (1963) and The Outer Limits (1995).
One of the first Brit actors to study the Method style of acting.
Of the three original stars of The Man from U.N.C.L.E. (1964), he is the only one who never appeared on the spin-off series The Girl from U.N.C.L.E. (1966).
He has a daughter, Sophie, and son, Peter, from his marriage to Katherine Carpenter. Jill Ireland and he had three children, Paul McCallum, Valentine McCallum, and an adopted son, Jason, who died on 7th November 1989 from an accidental drug overdose.