For nearly two decades, Mark Harmon became synonymous with television crime-solving through his role as Leroy Jethro Gibbs on NCIS. But according to the actor himself, one of the most important influences on his career never appeared on camera at all.
Harmon recently opened up about the real-life NCIS agent who helped shape both his performance and his understanding of the military investigative world behind the hit CBS series. What started as a professional collaboration eventually turned into a close friendship and creative partnership that continues even after Harmon stepped away from the long-running show.
Mark Harmon’s Agent Friendship Began Behind The Scenes
The Mark Harmon agent connection began when Harmon first met retired NCIS Special Agent Leon Carroll Jr., who joined the series as a technical advisor. Carroll spent more than 20 seasons helping the show maintain realism by sharing details from his real-life investigative experience.
According to Fox News, Harmon quickly became fascinated by Carroll’s background as both a Marine and an NCIS agent. The actor explained that he spent countless hours sitting beside Carroll between takes, asking questions and learning how agents actually approached their work. Harmon admitted he felt a major responsibility to portray the profession accurately, especially after developing deep respect for the people who perform those dangerous jobs in real life. He said Carroll’s insight completely changed the way he approached the role of Gibbs over the years.
Their Partnership Continued Long After NCIS
The Mark Harmon agent friendship eventually expanded beyond television. After years of working together on set, Harmon and Carroll teamed up as co-authors on several historical nonfiction books focused on military intelligence and real NCIS investigations. Their first collaboration, Ghosts of Honolulu, explored espionage and counterintelligence leading up to Pearl Harbor. They later followed it with Ghosts of Panama before releasing their latest project, Ghosts of Sicily, which examines organized crime and intelligence operations connected to World War II-era Sicily.
Even while discussing serious topics, both men admitted plenty of humor filled the NCIS set. Carroll joked that Harmon constantly played pranks during filming, while Harmon said humor helped the cast survive the show’s exhausting schedule. Although he left NCIS in 2021, Harmon still remains closely connected to the franchise through his friendship and writing partnership with Carroll.