Mark Harmon spent years chasing criminals on “NCIS”— but it was an off-screen encounter with a real agent that truly cracked open a turning point in his career.
The actor first met retired NCIS Special Agent Leon Carroll Jr. while starring as Leroy Jethro Gibbs on the hit CBS drama. The behind-the-scenes connection grew into a lasting friendship.
Carroll spent 21 seasons as the show’s technical advisor, bringing real-life field experience to the set, People magazine reported. He now continues his work with the franchise on “NCIS: Origins.”
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In 2023, the pair teamed up as co-authors for “Ghosts of Honolulu.” The book tells the true story of a Japanese spy and the Navy counterintelligence agent determined to stop him in the tense lead-up to the attack on Pearl Harbor. They followed it up with 2025’s “Ghost of Panama,” which dives into a case in Central America.
Their new book, “Ghost of Sicily,” explores a real NCIS investigation into the Mafia’s grip on the island. Working together once cameras stopped rolling was a no-brainer, Harmon insisted.
“From the very, very beginning, I met Leon, and I spent a lot of time with him on set,” Harmon told Fox News Digital. “Some of that time, sometimes a lot of that time, was sitting in two chairs watching the setup of a shot and talking about this agency. For me, it was a deep dig from the very first day.”
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“… And also, you got to understand that I’m talking to a former Marine,” the 74-year-old shared. “[But] there are no former Marines. They’re always Marines. And then also, an NCIS special agent. So, my respect for that is huge.”
Harmon said it was always “my responsibility” to portray an NCIS agent accurately, based on Carroll’s no-nonsense education.
“I’m humbled by my appreciation of Leon as an individual and what he’s done in his life and what he’s accomplished,” said Harmon. “And also, this TV show, in a different way, presented to this agency opportunities that were never apparent before.”
But it wasn’t always serious on set.
“He is a practical joker,” Carroll told Fox News Digital about Harmon. “There are a lot of different stories I can tell you about things that Mark was behind. Sometimes other people got blamed for it, but I know for a fact that the cookies that showed up in my cargo pants pockets when I would get home were placed there by Mark Harmon. He’ll say I took them from the craft services table. [Also,] the nails and bolts that ended up in my pockets.”
“[There are] practical reactions and answers for me on all of these things that make total sense,” chimed Harmon as he beamed.
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“But he is a consummate practical joker,” Carroll stressed.
There’s a good reason for his antics, said Harmon.
“If you don’t keep your humor on a set, on a sound stage — and in those days, in the beginning of this show, we were working 20-plus-hour days — and if people aren’t enjoying it at some point, you’d better find another way to do it because you’ve got to love your work,” he said. “That has to be part of the interpretation of what you’re putting out there every week. And Leon was certainly a part of that.”
But the men shifted gears as they set out on their next chapter. In their latest book, readers are transported to 1942 New York to learn how the Office of Naval Intelligence (ONI) turned to an unlikely ally — the mob — to protect America’s waterfronts, the synopsis shared.
“The book chronicles the ONI’s controversial alliance with Charles ‘Lucky’ Luciano, the daring missions of agents like Tony Marsloe and the pivotal role this operation played in both the war effort and the evolution of international crime,” it read.
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“I have to say, in my experience, it takes a thief to catch a thief,” Carroll explained. “I wasn’t really surprised that our people back in the ’40s were doing exactly what I hope they’re still doing today. And that is recruiting sources who are not necessarily the best people in terms of their character.
“But I think the surprise was that we were able to actually gather a whole organization of five families of the Mafia, three of which were actively involved in providing us with information that we needed both to defend against German U-boats … and then moving on to a broader prospect of what Lucky Luciano was able to provide us in Sicily and other parts of Italy.”
These days, Harmon has plenty of time to embark on new adventures. He stepped away from “NCIS” after 18 seasons in 2021, People magazine reported. According to the outlet, he reprised his character in a few episodes of “NCIS: Origins.”
When Harmon was asked what Carroll had taught him about NCIS work, Carroll squirmed, joking, “I’m concerned.”
“I just come at this with respect,” said Harmon. “This is a really hard job. Tough job. Lots of jobs are hard and tough. I get it. [But it’s also] a dangerous job. And what they’ve done for many years with really no accolades, these individuals who do this work don’t do it for the notoriety. They pride themselves on that. So, in some ways, a TV show about that does not help their work, but in other ways, it does help their work because people know who they are now.”
“I do have to take responsibility for that because I’m an actor who played a role, and I dug deep when we first started,” he reflected. “I tried to continue to do that to grow the characters. I really look at this time now, and us working together here with this third book, as being an extension of that, just in a different realm.”
Harmon said he’s not done writing this new chapter with his pal.
“Am I surprised that [our friendship has] extended now to writing and to now publishing a third book? Absolutely,” said Harmon. “Do I think there’s something beyond this? Who knows? But right now, this is still part of a story to tell.”
