CBS did not replace Mark Harmon as Leroy Jethro Gibbs when NCIS: Origins launched. Instead, the prequel cast Austin Stowell as the younger version of Gibbs, a role that had previously been played in NCIS flashbacks by Harmon’s son, Sean Harmon. Sean remained closely involved with the franchise as an executive producer rather than continuing to portray the character.
The distinction is important because NCIS: Origins now uses two actors to show Gibbs at different points in his life. Stowell plays Gibbs in the early 1990s, when he is beginning his career as a special agent at the Naval Investigative Service office at Camp Pendleton. Mark Harmon plays the older, present-day Gibbs and has narrated the series since it began.
CBS announced Stowell’s casting in March 2024, months before the series premiered on October 14, 2024. The network said the story would begin in 1991 and follow Gibbs as a newly minted agent finding his place on a team led by Mike Franks.
Sean Harmon had already established himself as the younger Gibbs on the original NCIS. He appeared in flashback sequences that showed earlier moments from the character’s life. When the idea for a full prequel developed, however, the younger Harmon decided not to make the role his long-term acting job.
Mark Harmon later explained that his son had made a choice about the direction he wanted his career to take. Sean Harmon instead became one of the executive producers of NCIS: Origins, working on the series with his father and the show’s creative team.
That left producers searching for an actor who could carry an entire series as Gibbs rather than appear only in occasional flashbacks. Stowell won the part after an audition process in which the producers were looking for someone who could suggest the familiar Gibbs without simply copying Mark Harmon’s performance.
Stowell’s version of Gibbs starts from a very different place than the veteran agent viewers knew on NCIS. The prequel begins after the murders of Gibbs’ wife, Shannon, and daughter, Kelly, events that became a defining part of the character’s history on the original series. The younger Gibbs is grieving, uncertain and still learning how to function as an investigator.
That approach gives Stowell room to play a version of the character who has not yet developed the calm authority associated with Mark Harmon’s portrayal. The series follows his relationships with Mike Franks, played by Kyle Schmid, and Special Agent Lala Dominguez, played by Mariel Molino, as well as other members of the Camp Pendleton team.
Sean Harmon has said the idea behind the prequel was to explore how Gibbs became the leader audiences eventually knew. His earlier work as young Gibbs helped connect the character’s past to the original series, but his role on NCIS: Origins shifted to helping shape that history behind the scenes.
Mark Harmon also stayed involved from the start. He served as an executive producer and narrator during the first two seasons and appeared onscreen in selected episodes. His participation became much larger in Season 3.
CBS announced in June 2026 that Mark Harmon would appear as the older Gibbs in every episode of the third season. The season premiered October 6, 2026, and introduced a present-day mystery connected to Gibbs’ earlier years at Camp Pendleton. Stowell continued to play the younger Gibbs in the 1990s storyline.
The structure means NCIS: Origins is no longer simply a prequel told entirely through memories. Season 3 moves between the younger Gibbs and the retired version of the character, allowing Stowell and Harmon to portray different stages of the same life within the same season.
The October 20 episode, “Return of the Mack,” expanded that present-day story by reuniting Harmon’s Gibbs with Michael Weatherly’s Tony DiNozzo. CBS said Gibbs turns to DiNozzo, now serving as NCIS director, for help identifying the people who shot him. The episode also brought back Adam Campbell as a younger Donald “Ducky” Mallard in the 1992 storyline.
The casting history can therefore be confusing when it is described simply as one Gibbs actor “replacing” another. Three performers are tied to the character’s different eras: Mark Harmon originated the adult Gibbs on NCIS, Sean Harmon portrayed younger versions of his father’s character in flashbacks, and Austin Stowell became the full-time young Gibbs when NCIS: Origins was created.
Sean Harmon’s departure from the acting role did not represent a break from the project. He helped originate the prequel concept and remains credited as an executive producer. His decision allowed the series to choose a new actor for a much larger version of young Gibbs while keeping the Harmon family directly involved.
Stowell has now played Gibbs across all three seasons of NCIS: Origins. The current season also gives Mark Harmon his largest onscreen role in the franchise since his regular run on NCIS ended, creating a format in which the character’s past and present can develop side by side.
NCIS: Origins currently airs Tuesdays on CBS and streams on Paramount+. The series is no longer awaiting a debut or operating in early development: it is in its third season, with Austin Stowell continuing as young Gibbs and Mark Harmon appearing as the older Gibbs throughout the season.