Commander Samuel H. Brennan Jr. was born in Jacksonville, Florida on 13 February 1942. He enlisted in the United States Navy in 1959 and served in the destroyers USS PORTERFIELD (DD 682), USS BRINKLEY BASS (DD 887) and USS HIGBEE (DD 806) and the guided missile destroyer USS JOSEPH STRAUSS (DDG 16) while advancing to the rank of Chief Sonar Technician at 25 years old. He was selected for the Naval Enlisted Scientific Program (NESEP) and, after receiving the degree of Bachelor of Science in Electrical Engineering from the University of New Mexico, was commissioned an ensign in 1970.

Following commissioning, Commander Brennan was assigned to the USS CONNOLE (FF 1156) as Sonar Maintenance Officer and ASW Officer, prior to "fleeting up'' for a full tour as Weapons Officer. After leaving CONNOLE, he was assigned as First Lieutenant and Combat Cargo Officer on board USS FAIRFAX COUNTY (LST 1193). Commander Brennan next attended the Naval Postgraduate School in Monterey, California, where he earned a Master of Science Degree in Systems Technology, specializing in ASW Systems.

Upon completion of postgraduate studies, Commander Brennan served as Engineering Officer on board USS PHARRIS (FF 1094). His next assignment was as ASW Officer and Deputy SHAREM Officer at Surface Warfare Development Group, Little Creek, Virginia. Commander Brennan then served as Executive Officer, USS McCANDLESS (FF 1084), followed by a tour as Force ASW Officer on the staff of the Commander, Naval Surface Force, U.S. Atlantic Fleet. In September 1987, he was assigned as Naval Advisor, SACLANT Undersea Research Centre (SACLANTCEN), La Spezia, Italy and SACLANT Liaison Officer. SACLANTCEN is a NATO facility specializing in Underwater acoustics and Oceanographic research, as well as ASW Systems development. Commander Brennan reported to his last assignment as Weapons Engineering Curricular Officer (at the Naval Postgraduate School in Monterey, California) in September 1990. He retired from the Navy on 1 August 1992.