NCIS has been on the air since 2003 - and the fan favorite drama is still going strong. The CBS series follows a group of special agents from the Naval Criminal Investigative Service as they investigate various different crimes. NCIS season 1 began with lead actors Mark Harmon, Sasha Alexander, Michael Weatherly, Pauley Perrette and the late David McCallum, however, the cast has consistently changed over the 20-plus seasons as actors have come and gone. Harmon - who played Leroy Jethro Gibbs - left NCIS after 18 seasons in October 2021.