Leroy Jethro Gibbs has worked his last case. Monday's episode of NCIS was Mark Harmon's final one as the lead of the long-running CBS procedural. He exits the show after 18 years of playing Gibbs, dating back to the character's introduction on JAG in 2003. Monday's episode, the fourth of Season 19, concluded with Gibbs - who was nearly killed in a boat explosion in the Season 18 finale - opting not to return to his job in NCIS.