Mark Harmon Credits NCIS Agent Friend for Shaping Gibbs Role and Co-Authored Books

Mark Harmon met retired NCIS Special Agent Leon Carroll Jr. while playing Leroy Jethro Gibbs on the CBS drama NCIS. Their connection grew into a friendship, with Carroll serving 21 seasons as the show's technical advisor, drawing on his field experience. He continues that role on NCIS: Origins, People magazine reported.

In 2023, Harmon and Carroll co-authored Ghosts of Honolulu, recounting a Japanese spy and a Navy counterintelligence agent before the Pearl Harbor attack. They followed with Ghost of Panama in 2025, covering a Central America case. Their newest, Ghost of Sicily, examines a real NCIS investigation into the Mafia's hold on the island.

Working together off-camera felt natural, Harmon said. "From the very, very beginning, I met Leon, and I spent a lot of time with him on set," he 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."

"And also, you got to understand that I'm talking to a former Marine," the 74-year-old added. "There are no former Marines. They're always Marines. And then also, an NCIS special agent. So, my respect for that is huge."

Harmon felt a responsibility to depict an NCIS agent accurately, guided by Carroll's direct input. "I'm humbled by my appreciation of Leon as an individual and what he's done in his life and what he's accomplished," he said. "And also, this TV show, in a different way, presented to this agency opportunities that were never apparent before."

Set life stayed lighthearted. "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," Harmon replied.

"But he is a consummate practical joker," Carroll said.

Harmon explained the pranks had purpose. "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."

Their latest book shifts to 1942 New York, where the Office of Naval Intelligence allied with the mob to guard waterfronts. The synopsis notes the ONI's controversial partnership with Charles 'Lucky' Luciano, missions by agents like Tony Marsloe, and the operation's impact on the war and international crime.

"I have to say, in my experience, it takes a thief to catch a thief," Carroll said. "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."

Harmon left NCIS after 18 seasons in 2021 but appeared in episodes of NCIS: Origins, People reported. Asked what he learned from Carroll about NCIS work, Carroll joked, "I'm concerned."

"I just come at this with respect," Harmon said. "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 added. "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 plans to keep writing with his friend.