Mark Harmon on the Real NCIS Agent Who Made Gibbs

Mark Harmon played Leroy Jethro Gibbs for nearly two decades — but the man who truly built that character wasn’t a writer or a director. He was a real NCIS special agent who showed up on set every day and sat down in the chair next to Harmon while the cameras were being set up.

That man is retired NCIS Special Agent Leon Carroll Jr., and Harmon is the first to tell you: he owes him a lot.

“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.”

Carroll spent 21 seasons as the technical advisor on the hit CBS drama, bringing genuine field experience to every scene. He now continues that work on NCIS: Origins, the prequel series following a young Gibbs — played by Austin Stowell — through the early years that shaped the man Harmon would eventually bring to life. The connection between the actor and the agent, though, goes far beyond the set.

From the Set to the Bookshelf

In 2023, Harmon and Carroll co-authored Ghosts of Honolulu, a nonfiction account of a Japanese spy and the Navy counterintelligence agent racing to stop him before Pearl Harbor. They followed it with Ghost of Panama in 2025. Now comes their third collaboration, Ghost of Sicily — a deep dive into a real NCIS investigation into the Mafia’s iron grip on the island.

The new book takes readers back to 1942 New York, tracing how the Office of Naval Intelligence made a controversial deal with an unlikely partner: organized crime. Specifically, the five families — three of which actively fed the ONI intelligence used to defend against German U-boats and, eventually, to support the Allied push into Italy.

“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.”

The book centers on Charles “Lucky” Luciano’s role in that alliance, alongside the daring field work of agents like Tony Marsloe — real history that Carroll says reflects the messy, high-stakes reality of intelligence work he knows firsthand.

What the Real Agent Taught the Actor

Harmon has always been open about the weight he felt playing an NCIS agent. He saw it as a responsibility — not just to the character, but to the real people the show represented.

“I’m humbled by my appreciation of Leon as an individual and what he’s done in his life and what he’s accomplished,” Harmon said. “And also, this TV show, in a different way, presented to this agency opportunities that were never apparent before.”

He didn’t take that lightly. Carroll is a former Marine — “there are no former Marines,” Harmon was quick to clarify, “they’re always Marines” — and a decorated agent whose career demanded a kind of respect that Harmon says shaped every scene he filmed as Gibbs.

“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.”

It’s a tension he acknowledged openly: a TV show about a secretive agency simultaneously helps and hurts the real people doing that work. But Harmon says he tried to honor it. “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. I tried to continue to do that to grow the character.”

The Lighter Side of a Long Friendship

Twenty-plus-hour shooting days in the early seasons of NCIS could have made the whole thing grim. Instead, Harmon made sure it wasn’t.

“He is a practical joker,” Carroll said of his co-author with a laugh. “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.”

Harmon didn’t exactly deny it. “[There are] practical reactions and answers for me on all of these things that make total sense,” he said, beaming.

“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,” Harmon 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.”

Life After Gibbs — and What Comes Next

Harmon stepped away from NCIS after 18 seasons in 2021, though he returned to reprise Gibbs in a handful of episodes of NCIS: Origins — including a Season 2 appearance where fans finally saw the older Gibbs hunkered down in his Alaskan home, a stray dog wandering in from the woods and staying for company. (When asked on the official NCIS: Partners & Probies podcast whether the dog had a name, Harmon was charmingly noncommittal: “I just know he’s got a friend sometimes.”)

The prequel series has been busy filling in the mythology fans have always wondered about — including, in its Season 2 finale, the moment Mike Franks first slapped a young Gibbs upside the head, the origin of that iconic gesture, and how the Naval Investigative Service rebranded itself into the Naval Criminal Investigative Service. The history runs deep, and so does Harmon’s connection to it.

As for what’s next for him and Carroll — the answer is deliberately open-ended.

“Am I surprised that [our friendship has] extended now to writing and to now publishing a third book? Absolutely,” Harmon said. “Do I think there’s something beyond this? Who knows? But right now, this is still part of a story to tell.”