NCIS Star Mark Harmon Admits He Doesn't Know Gibbs' Iconic Rules

In November 2025, it was revealed that Mark Harmon, the actor who portrayed Leroy Jethro Gibbs on the long-running NCIS series for nearly 20 years, does not actually know the iconic list of rules his character created over the course of the show. This surprising admission came to light during a recent NCIS crossover episode that dealt specifically with one of Gibbs' rules, Rule No. 11: "When the job's over, walk away."

Gibbs' rules were a key part of the character and a beloved aspect of the NCIS franchise, which has spawned multiple spinoffs over the years. Fans have long been fascinated by the rules and would often ask Harmon about them. However, according to NCIS: Origins co-creator David J. North, Harmon hasn't actually kept any of the rules committed to memory. The rules were simply lines in a script that he read and then moved on from.

This revelation is sure to disappoint many NCIS fans, who likely assumed that after playing the role of Gibbs for nearly two decades, Harmon would have the rules memorized. The recent crossover episode, which saw a case from Gibbs' early career resurface in the modern NCIS team, was a rare return appearance for Harmon since he left the show in 2021.