The MCRT is led by Supervisory Special Agent Leroy Jethro Gibbs. Gibbs’ team is composed of Special Agent and Senior Field Agent Anthony "Tony" DiNozzo, Special Agent Timothy McGee, and ex-NSA analyst and probationary Special Agent Eleanor Bishop. Previous members include Special Agent (former Mossad liaison officer) Ziva David until she returned to Israel in season eleven, and Caitlin "Kate" Todd who was shot and killed by rogue Mossad agent Ari Haswari, Ziva's half-brother, at the end of season two. The team is assisted in their investigations by Chief Medical Examiner Dr. Donald "Ducky" Mallard, his assistant Jimmy Palmer, who replaced Gerald Jackson, and Forensic Specialist Abigail "Abby" Sciuto.
It is revealed, through flashbacks, that the head of the MCRT before Gibbs was Special Agent Mike Franks (Muse Watson), who led the unit when it was part of the Naval Investigative Service (NIS), the predecessor agency of the NCIS. Franks recruited Gibbs shortly after Gibbs' retirement from the Marine Corps, eventually retiring himself some years later. After Franks' departure, Gibbs recruited DiNozzo from the Baltimore Police Department's Homicide Section. The two were briefly joined by Vivian Blackadder (Robyn Lively), whom Gibbs recruited from the FBI. In the second part of the NCIS pilot, Blackadder allowed her emotions to nearly derail an anti-terror operation in Spain. Gibbs is noticeably disappointed; Blackadder is not present in the series' first regular episode, instead being replaced by Caitlin Todd, a Secret Service agent who joins Gibbs' team after resigning from the Secret Service. Todd is eventually killed in the line of duty in the season two finale, and is ultimately replaced by Ziva David, who from season three to season seven is the Mossad liaison officer, and eventually gains American citizenship to become a full agent, David returns to Israel in the beginning of season 11, and is soon replaced by NSA Analyst, Eleanor Bishop. McGee first appears as a Field Agent assigned to the Norfolk Field Office. He uses his computer skills to aid the MCRT in subsequent investigations through the rest of season one, until he is officially promoted with his own desk at the Navy Yard in the beginning of season two.