Samuel Jackson Underhill was born on 25 August 1917 in Jericho, New York. He received a Bachelor of Science from Yale University and attended Harvard Law School before enlisting in the United States Navy Reserve as a Seaman Second Class on 8 November 1940. After serving briefly at Floyd Bennett Field, Brooklyn, New York, Underhill was appointed an aviation cadet and was transferred on 6 February 1941 to the Naval Air Station, Jacksonville, Florida, for flight training. He subsequently underwent further training at Miami, Florida; was designated a naval aviator on 15 July; and was commissioned an ensign in the Naval Reserve on 6 August. Following advanced flight training at Norfolk, Virginia, he reported to Scouting Squadron 5 (VS-5).