Leon Carroll grew up in Chicago and graduated from North Dakota State University (where he played on college Division II National Championship football teams). He served 6 years in the Marines and then continued in the Marine Reserves in Long Beach (under the command of PV Sunset member Lt Col Jacques Naviaux).

Carroll was a member of the U.S. Marine Corps when he joined NCIS in 1980. Leon worked at a halfway house for pre-release felons in Fargo, North Dakota, and then became a special agent with the Naval Investigative Service, later known as the Naval Criminal Investigative Service (NCIS), serving in several places including Panama and aboard the USS Ranger.

He retired two decades later, but returned after 9/11, serving another year and a half to help with the agency's expanded role in counter-terrorism. After his second retirement, he and his wife moved to the Los Angeles area.

As a retired NCIS agent with over 20 years of experience, Mr. Carroll received an unexpected opportunity to work as a technical adviser to the NCIS TV program in Los Angeles. He was recruited in 2003 by the producers who said they needed someone who could provide the show with the “spit-polish shine of authenticity.”

Working on both NCIS and NCIS Los Angeles, Mr. Carroll is a technical adviser to the script writers, actors and director, and has also written scripts for a few episodes himself. He works under the leadership of Mark Hyman of football fame. They do 24 episodes per season.