Thomas Sullivan Magnum IV was born and raised in Norfolk, Virginia. His mother, Katherine Magnum, was a homemaker. His father, Thomas Sullivan Magnum III, was an airline pilot and former U.S. Navy fighter pilot. After Magnum graduated from high school, he began attending the United States Naval Academy. He graduated from the Naval Academy with a Bachelor of Science in Mechanical Engineering and a commission as an Ensign in the United States Navy.
While at the Academy, Magnum decided to become a Navy SEAL. After Basic Underwater Demolition/SEAL (BUD/S) Training and basic parachute training at the U.S. Army Airborne School at Fort Benning, he went on to SEAL Qualification Training. Following his completion of SQT, Magnum attended SEAL Delivery Vehicle School and was then assigned as an assistant platoon commander to SEAL Delivery Vehicle Team 1 at Naval Station Pearl Harbor in Hawaii. After his tour with SDVT-1, he was assigned as a platoon commander to SEAL Team 1 at Naval Amphibious Base Coronado in California.
While with SEAL Team 1, Magnum earned numerous commendations. Because of that, recruiters for the United States Naval Special Warfare Development Group ("DEVGRU") took notice of his abilities and approached him about joining their ranks. Magnum then went on to the NSWDG selection and training program at Dam Neck, Virginia. After successfully completing the arduous training process, he joined DEVGRU (aka "SEAL Team 6"). As an officer in SEAL Team 6, Magnum served as a troop commander in Blue Squadron.
After twelve years of distinguished service, he left the Navy and decided to return to Hawaii. Magnum has become a private investigator "beach bum" who, despite irregular employment, nevertheless manages to live a comfortable existence thanks to celebrity author Robin Masters's offer of the guest house on his Hawaii estate "Robin's Nest" and use of his red Ferrari 308 GTS in exchange for quality control of the estate security.
Tom Magnum and his friends are frequently involved not only in typical "P.I." cases but also a wide variety of adventures involving espionage, covert operations, paramilitary escapades, and "lifestyles of the rich and famous."