The franchise star left the series four episodes into season 19.
Harmon departed the long-running CBS procedural four episodes into the 19th season last fall, with his final hour airing Oct. 11, 2021. Though that episode marked Harmon's last onscreen appearance on NCIS, he continued to lead the show's opening credits for the rest of the season.
On Monday's season 20 opener, which kicked off a two-hour crossover event with NCIS: Hawaii, that all changed, marking the official end of an era. Sean Murray, who plays McGee, now leads the show's opening credits, followed by co-stars Wilmer Valderrama, Katrina Law, Brian Dietzen, Diona Reasonover, David McCallum, Rocky Carroll and Gary Cole.
Months after his departure, Harmon opened up for the first time about his exit from the franchise as part of a special featurette celebrating Gibbs' legacy.
"What has always drawn me here is the character I play and to keep it fresh and to keep it challenging," the 70-year-old actor said, before alluding to the fact that it was time to close this chapter in his career for now. "Plot-wise, this character has taken the path that it did. I thought it was honest and OK with."
He assured viewers Gibbs is enjoying the simpler things in life: "I'm not retired... The character is living in Alaska as far as I know."
In Harmon's final episode, Gibbs broke the news to McGee -- while out fishing on the lake in Alaska -- that he was not returning to work, acknowledging that he was "not sure" how long he'd be away. He then passed the baton to McGee. "I could not have hoped for anyone better to watch my back for the past 18 years than you," he said to McGee.
“As an executive producer and dear friend, Mark continues to be an integral part of the fabric of the show. Our north star has always been staying true to our characters, and that truth has always guided the stories we tell and where those characters go," executive producer and showrunner Steven D. Binder said following Harmon's exit. "So regarding the future of Gibbs, as longtime fans of the show may have noticed over the years… never count Leroy Jethro Gibbs out.”
NCIS airs Mondays at 9 p.m. ET/PT on CBS.