‘NCIS: Origins’: Sean Harmon to Guest Star — Who’s He Playing?

Meredith Jacobs September 21, 2026, 3:48 pm

Sean Harmon NCIS Episode 400 Young Gibbs — CBS

What To Know

Sean Harmon, who previously played young Gibbs in NCIS flashbacks, will guest star in NCIS: Origins Season 3.
Find out details about who he’ll be playing and when as well as what else to expect from that episode.

When NCIS did flashbacks involving a younger Gibbs, Mark Harmon‘s son, Sean Harmon, played the character in eight episodes (including opposite Adam Campbell as the young version of David McCallum‘s Ducky, pictured above and below). However, when the prequel, NCIS: Origins, debuted two years ago, Austin Stowell stepped into the young Leroy Jethro Gibbs’ shoes for the ’90s storyline. Mark Harmon narrates every episode, executive produces, and will appear onscreen in every Season 3 episode. But now Sean Harmon will be appearing on the prequel.

CBS released the description for Season 3 Episode 2, titled “My Own Worst Enemy,” airing Tuesday, October 13. It reads: “In the present, Gibbs (Mark Harmon) returns to civilization and calls in a favor from old friend Woody (Bobby Moynihan). In 1992, the team bands together to help Woody when he’s accused of murdering a bowling rival.”

It also reveals that Sean Harmon, who serves as an executive producer, will be guest-starring as FBI Agent Casey Bloom.

Adam Campbell as young Donald Mallard and Sean Harmon as young Leroy Jethro Gibbs in NCIS Episode 400 - 'Everything Starts Somewhere'

CBS

Mark explained to Variety ahead of the series premiere in 2024 why Sean isn’t playing young Gibbs again: “He’s an actor, writer, producer, a surfer, he does a lot of things. You have to come to decisions about what you want to do in life, or what you think’s important. I just think he made a choice and had to make a decision in some ways. He likes executive producing.”

'NCIS: Origins' Boss & Mark Harmon on His Return, What's Next for Gibbs and Lala

We already knew that Moynihan once again dons the aged-up makeup in Season 3; he previously appeared as present-day Woody alongside Ely Henry‘s Phil in the NCIS and Origins crossover last season. The Season 3 trailer shows Woody visiting Mark’s Gibbs in Alaska.

“I came to Alaska to find peace, but ghosts from my past found me. I know now peace never lasts,” Mark’s Gibbs says in voiceover in the video. “You’re doing me a favor,” he says to Woody.

NCIS: Origins, Season 3 Premiere, Tuesday, October 6, 10/9c, CBS