Since David McCallum's father, David McCallum Sr., was first violinist for the London Philharmonic Orchestra and his mother, Dorothy Dorman, was a cellist, it's not surprising that David was originally headed for a career in music, playing oboe. He studied briefly at the Royal Academy of Music. He left that, however, for the Royal Academy of Dramatic Art and joined Actor's Equity in 1946, his first acting work being for BBC Radio. He made nearly a dozen movies in the United Kingdom before his critically acclaimed work as Lt. Wyatt in Billy Budd (1962).

To the older generation, he is perhaps best known for his portrayal of U.N.C.L.E. agent Illya Kuryakin in the hit TV series The Man from U.N.C.L.E. (1964). To younger audience, he is best known for his superlative portrayal of Dr. Donald "Duckie" Mallard on NCIS: Naval Criminal Investigative Service (2003).

McCallum was first married to actress Jill Ireland, whom he met while filming Hell Drivers (1957). In 1962 he introduced Ireland to Charles Bronson when both were filming The Great Escape (1963). She eventually left McCallum and married Bronson in 1968. McCallum and Ireland had three sons: Paul, Jason (an adopted son who died from an accidental drug overdose in 1989), and Val (short for Valentine).

He met fashion model Katherine Carpenter during a photo shoot for Glamour in 1965 and married her two years later. The couple had a son, Peter, and a daughter, Sophie. They were together for 58 years and were active with charitable organizations that support the The United States Marine Corps: Katherine's father was a Marine who served in the Battle of Iwo Jima, and her brother lost his life in the Vietnam War. McCallum had eight grandchildren.

David McCallum died on September 25 2023 in New York City from natural causes at the age of 90.