NCIS’s Mark Harmon Reveals His Favorite Gibbs Rule & It’s Surprisingly Practical

Leroy Jethro Gibbs had a lot of rules he followed on NCIS, and Mark Harmon, the actor who played him, has revealed his favorite among them.

Following his debut as Gibbs in the JAG two-parter “Ice Queen” and “Meltdown,” Harmon starred as Gibbs on NCIS for the show’s first 18 seasons and the first four episodes of Season 19. Over that nearly-20-year span, viewers of the popular CBS procedural learned about the many rules Gibbs had come up with for living his life. Of those dozens, the one Harmon appreciates most is also arguably the most practical.

Harmon appeared on Partners & Probies podcast alongside longtime NCIS technical advisor and former real life NCIS Special Agent Leon Carroll Jr. to promote their new book, Ghosts of Sicily. During their conversation with hosts Brian Dietzen and Diona Reasonover, who respectively play Jimmy Palmer and Kasie Hines on NCIS, Harmon responded to a fan question about his favorite Gibbs rule by pulling out a pocket knife and sharing his favortire rule:

[Rule 9,] carry a knife is a good one. You’d be surprised how useful it is. All the boys, they all carry a little pocket knife, and you can forget this sometimes, and you’re getting on an airplane, and then they take your little pocket knife, and it’s gone, which has happened, too. But it’s just a handy tool. I don’t mean to defend yourself, what I’m talking about, but oftentimes people are saying, “Anybody got a knife?”

Of the dozens of Gibbs rules that have been revealed over NCIS’s long run on CBS, carrying a knife definitely comes in handy, at least of the pocket variety. Like Harmon said, there are times when having one on hand is useful for either the person it belongs to or to loan to someone nearby. It doesn’t even have to be the blade part either, as the mini-scissors on a pocketknife are another practical addition.

In Gibbs’ case, though, his use of knives is more intense than simply cutting a thread or whittling some wood. As a former Marine and NCIS Special Agent, there were times on NCIS when the character was using various kinds of blades to cut tires or even in combat. So for him, carrying a knife is even more important than it would be for the average person.

These days, however, Gibbs is leading a much quieter life. After officially leaving the NCIS agency during season 18, Harmon’s character decided to stay in Naktok Bay, Alaska after completing one final mission with Timothy McGee and Alden Parker in “Great Wide Open.” Still, because he now lives in such a remote area, it still doesn’t hurt for him to have a knife on hand just to be safe.

Though Harmon has yet to reprise Gibbs on NCIS since his departure in 2021, he continues to play the character on NCIS: Origins. Along with executive producing duties, he narrates the prequel series as the older Gibbs, while Austin Stowell stars as the younger Gibbs. Harmon has also appeared on camera as Gibbs twice in NCIS: Origins; the first time was in the pilot, and the second time was in the season 2 episode “Funny How Time Slips Away.”

NCIS: Origins season 3 will air on CBSs 2026-2027 TV schedule, though it will have a smaller number of episodes compared to the previous two seasons. Nevertheless, perhaps one of these forthcoming 10 episodes could shed light on why it became so important for Gibbs to carry a knife in NCIS.