LOS ANGELES >> Actor David McCallum, who became a teen heartthrob in the hit series “The Man From U.N.C.L.E.” in the 1960s and was the eccentric medical examiner in the popular “NCIS” 40 years later, has died. He was 90.

McCallum died Monday of natural causes surrounded by family at New York Presbyterian Hospital, CBS said in a statement.

“David was a gifted actor and author, and beloved by many around the world. He led an incredible life, and his legacy will forever live on through his family and the countless hours on film and television that will never go away,” said a statement from CBS.

Scottish-born McCallum had been doing well appearing in such films “A Night to Remember” (about the Titanic), “The Great Escape” and “The Greatest Story Ever Told” (as Judas). But it was “The Man From U.N.C.L.E.” that made the blond actor with the Beatlesque haircut a household name in the mid-’60s.

The success of the James Bond books and films had set off a chain reaction, with secret agents proliferating on both large and small screens. Indeed, Bond creator Ian Fleming contributed some ideas as “The Man From U.N.C.L.E.” was being developed, according to Jon Heitland’s “The Man From U.N.C.L.E. Book.”

The show, which debuted in 1964, starred Robert Vaughn as Napoleon Solo, an agent in a secretive, high-tech squad of crime fighters whose initials stood for United Network Command for Law and Enforcement. Despite the Cold War, the agency had an international staff, with McCallum as Illya Kuryakin, Solo’s Russian sidekick.

The role was relatively small at first, McCallum recalled, adding in a 1998 interview that “I’d never heard of the word ‘sidekick’ before.”

The show drew mixed reviews but eventually caught on, particularly with teenage girls attracted by McCallum’s good looks and enigmatic, intellectual character. By 1965, Illya was a full partner to Vaughn’s character and both stars were mobbed during personal appearances.

The series lasted to 1968. Vaughn and McCallum reunited in 1983 for a nostalgic TV movie, “The Return of the Man From U.N.C.L.E.,” in which the agents were lured out of retirement to save the world once more.

McCallum returned to television in 2003 in another series with an agency known by its initials — CBS’ “NCIS.” He played Dr. Donald “Ducky” Mallard, a bookish pathologist for the Naval Criminal Investigation Service, an agency handling crimes involving the Navy or the Marines. Mark Harmon played the NCIS boss.

McCallum said he thought Ducky, who sported glasses and a bow tie and had an eye for pretty women, “looked a little silly, but it was great fun to do.” He took the role seriously, too, spending time in the Los Angeles coroner’s office to gain insight into how autopsies are conducted.

Co-star Lauren Holly took to X, formerly Twitter, to mourn: “You were the kindest man. Thank you for being you.” The previously announced 20th anniversary “NCIS” marathon on Monday night will now include an “in memoriam” card in remembrance of McCallum.

The series built an audience gradually, eventually reaching the roster of top 10 shows. McCallum, who lived in New York, stayed in a one-bedroom apartment in Santa Monica when “NCIS” was in production.

“He was a scholar and a gentleman, always gracious, a consummate professional, and never one to pass up a joke. From day one, it was an honor to work with him and he never let us down. He was, quite simply, a legend, said a statement from “NCIS” Executive Producers Steven D. Binder and David North.

McCallum’s work with “U.N.C.L.E.” brought him two Emmy nominations, and he got a third as an educator struggling with alcoholism in a 1969 Hallmark Hall of Fame drama called “Teacher, Teacher.”

In 1975, he had the title role in a short-lived science fiction series, “The Invisible Man,” and from 1979 to 1982 he played Steel in a British science fiction series, “Sapphire and Steel.” Over the years, he also appeared in guest shots in many TV shows, including “Murder, She Wrote” and “Sex and the City.”

He appeared on Broadway in a 1968 comedy, “The Flip Side,” and in a 1999 revival of “Amadeus” starring Michael Sheen and David Suchet. He also was in several off-Broadway productions.

Largely based in the U.S. from the 1960s onward, McCallum was a longtime American citizen, telling The Associated Press in 2003 that “I have always loved the freedom of this country and everything it stands for. And I live here, and I like to vote here.”

David Keith McCallum was born in Glasgow in 1933. His parents were musicians; his father, also named David, played violin, his mother played cello. When David was 3, the family moved to London, where David Sr. played with the London Philharmonic and Royal Philharmonic.

Young David attended the Royal Academy of Music where he learned the oboe. He decided he wasn’t good enough, so he turned to theater, studying briefly at the Royal Academy of Dramatic Art. But “I was a small, emaciated blond with a caved chest, so there weren’t an awful lot of parts for me,” he commented in a Los Angeles Times interview in 2009.

After time out for military service, he returned to London and began getting work on live television and movies, In 1957 he appeared in “Robbery Under Arms,” an adventure set in early Australia, with a rising actress, Jill Ireland. The couple married that same year.

In 1963, McCallum was part of the large cast of “The Great Escape” and he and his wife became friendly with Charles Bronson, also in the film. Ireland eventually fell in love with Bronson and she and McCallum divorced in 1967. She married Bronson in 1968.

“It all worked out fine,” McCallum said in 2009, “because soon after that I got together with Katherine (Carpenter, a former model) and we’ve been very happily married for 42 years.”

McCallum had three sons from his first marriage, Paul, Jason and Valentine, and a son and daughter from his second, Peter and Sophie. Jason died of an overdose.

“He was a true Renaissance man — he was fascinated by science and culture and would turn those passions into knowledge. For example, he was capable of conducting a symphony orchestra and (if needed) could actually perform an autopsy, based on his decades-long studies for his role on NCIS,” Peter McCallum said in a statement.

In 2007, when he was working on “NCIS,” McCallum told a reporter: “I’ve always felt the harder I work, the luckier I get. I believe in serendipitous things happening, but at the same time, dedicating yourself to what you do is the best way to get along in this life.”