In film roles, McCallum notably appeared in The Great Escape (1963), and as Judas Iscariot in The Greatest Story Ever Told (1965).

Early in the Second World War, he was evacuated back to Scotland, where he lived with his mother at Gartocharn by Loch Lomond.

[8] Also involved in local amateur drama, at age 17, he appeared as Oberon in an open-air production of A Midsummer Night's Dream with the Play and Pageant Union.

He joined the British Army's 3rd Battalion the Middlesex Regiment, which was seconded to the Royal West African Frontier Force.

He began his acting career doing boy voices for BBC Radio in 1947 and taking bit parts in British films from the late 1950s.

[14] His early roles included an outlaw in Robbery Under Arms, (1957) junior RMS Titanic radio operator Harold Bride in A Night to Remember (1958), and a juvenile delinquent in Violent Playground (1958).

His first American film was Freud: The Secret Passion (1962),[15] directed by John Huston, which was shortly followed by a role in Peter Ustinov's Billy Budd.

Although the show aired at the height of the Cold War, McCallum's Russian alter ego became a pop culture phenomenon.

[13] While playing Kuryakin, McCallum received more fan mail than any other actor in Metro-Goldwyn-Mayer's history, including such popular MGM stars as Clark Gable, Robert Taylor, and Elvis Presley.

[17] Hero worship even led to a record, "Love Ya, Illya", performed by Alma Cogan under the name Angela and the Fans, which was a pirate radio hit in Britain in 1966.

McCallum received two Emmy Award nominations in the course of the show's four-year run (1964–1968) for playing the intellectual and introverted secret agent.

In an interview for a retrospective television special, McCallum recounted a visit to the White House during which, while he was being escorted to meet the U.S. president, a Secret Service agent told him, "You're the reason I got this job.

"[19] McCallum never quite repeated the popular success he had gained as Kuryakin until NCIS, though he did become a familiar face on British television in such shows as Colditz (1972–1974), Kidnapped (1978), and ITV's science-fiction series Sapphire & Steel (1979–1982) opposite Joanna Lumley.

In season 1 of seaQuest DSV, he appeared as the law-enforcement officer Frank Cobb of the fictional Broken Ridge of the Ausland Confederation, an underwater mining camp off the coast of Australia by the Great Barrier Reef; he also had a guest-star role in one episode of Babylon 5 as Dr. Vance Hendricks in the Season 1 episode "Infection".

[20] According to the behind-the-scenes feature on the 2006 DVD of NCIS season 1, McCallum became an expert in forensics to play Mallard, including attending medical examiner conventions.

In the feature, Donald P. Bellisario says that McCallum's knowledge became so vast that at the time of the interview, he was considering making him a technical adviser to the show.

McCallum appeared at the 21st Annual James Earl Ash Lecture, held 19 May 2005 at the Armed Forces Institute of Pathology, an evening for honouring America's service members.

[26] With series lead Mark Harmon's departure from the show in the fall of 2021 (Season 19), McCallum became the last remaining member of the original NCIS cast until his death in 2023.

It begins with Medical Examiner Jimmy Palmer finding Ducky had died at his home, the morning before he is set to help a young woman clear her USMC father's name of desertion.

McCallum's version of "The Edge" appears on the soundtracks to the 2008 video game Grand Theft Auto IV and the 2017 film Baby Driver.

As a classically trained musician, he conceived a blend of oboe, cor anglais, and strings with guitar and drums, and presented instrumental interpretations of hits of the day.

On Open Channel D, McCallum did sing on the first four tracks, "Communication", "House on Breckenridge Lane", "In the Garden, Under the Tree" (the theme song from the film Three Bites of the Apple), and "My Carousel".

[32] On 16 September 1967, McCallum married fashion model-turned-interior designer Katherine Carpenter in Valley Stream, New York.