Stephen Henry Ambruster was born on 14 December 1904, in St. Louis, Missouri, son of Harrison N. and Zelphia LaVenture Ambruster. He was graduated from Central High School in St. Louis in 1922, and entered the US Naval Academy, Annapolis, Maryland, from the State of Missouri on 16 June 1924. Graduated and commissioned Ensign on 7 June 1928, he subsequently progressed to the rank of Captain on 1 December 1950, to date from 25 March 1945. Upon his transfer to the Retired List on 1 July 1958, he was advanced to the rank of Rear Admiral, on the basis of combat citations.
After graduation from the Naval Academy in June 1928, he served for three years in USS West Virginia, flagship of Battleship Divisions, Battle Fleet. From July to December 1931 he had instruction at the Submarine School, New London, Connecticut, and in January 1932 joined USS Nautilus, of Submarine Division 12, Submarine Force. Detached in June 1935, he had two years' duty at the US Naval Ammunition Depot, West Loch, Oahu, Territory of Hawaii. That tour was followed by successive service at sea until July 1941 in USS Saratoga, flagship of Aircraft, Battle Force, and USS Perch, flagship of Submarine Squadron 6, and later Submarine Division 14, Submarine Force.
During the period August 1941 to April 1942 he was attached to the Submarine Base, Pearl Harbor, TH, and for a year thereafter served as Commanding Officer of USS Tambor, of Submarine Squadron 6. Under his command that submarine participated in four war patrols from Western Australia. For "conspicuous gallantry and intrepidity as Commanding Officer of the USS TAMBOR in action against enemy Japanese forces during the period July 24, 1942 to April 14, 1943..." he was awarded the Silver Star Medal. The citation continues: "Despite the strain of prolonged undersea operations in perilous waters, (he) pressed home his relentless attacks...sinking three hostile ships totaling 22,000 tons, and damaging three additional vessels totaling 17,500 tons...successfully completed his dangerous assignments by bringing his undersea craft to safety without having sustained disabling damage..."
Returning to the United States in June 1943, he fitted out USS Robalo (SS-273) and assumed command at her commissioning on 28 September of that year. He commanded that submarine in one war patrol before he was detached in April 1944 for brief staff duty with Commander Submarine Squadron 16. The same month he reported for duty as Chief of Staff to Commander Naval Operating Base, Midway Island, in which capacity he served until July 1945. Returning to the United States, he reported in August 1945 to the University of Idaho, at Moscow, where he had a three year tour as Professor of Naval Science, Naval Reserve Officers Training Corps Unit. For one year thereafter he commanded USS Monongahela (A0-42), and in August 1949 reported to the Bureau of Naval Personnel, Navy Department, Washington, DC, for duty as Director of the Personnel Affairs Division.
He commanded USS Arcadia from August 1952 until August 1953, then reported as Assistant Chief of Staff for Personnel to the Commandant, Thirteenth Naval District, with Headquarter in Seattle, Washington. In July 1956 he was transferred to Headquarters, Eleventh Naval District, San Diego, California, where he served until released from all active duty pending his retirement on 1 July 1958.
In addition to the Silver Star Medal, Rear Admiral Ambruster had the American Defense Service Medal; the American Campaign Medal; the Asiatic-Pacific Campaign Medal; the World War II Victory Medal; and the National Defense Service Medal. He also wore the Submarine Combat Pin.