Following graduation from the Naval Academy in 1910, Pownall served briefly in the USS Mississippi (BB-23), USS Missouri (BB-11), USS Ammen (DD-35), and USS Reid (DD-21). During World War I he commanded the USS Roe (DD-24) and USS Vedette (SP-163), and after the war, the USS John D. Ford (DD-228). After flight training, he was designated a naval aviator in 1927. He then served as navigator in the USS Saratoga (CV-3), air officer in the USS Lexington (CV-2), and had duty on the staffs of Commander in Chief Battle Fleet and Commander Aircraft Squadrons Battle Fleet. He was executive officer of the USS Ranger (CV-4), and skipper of the USS Enterprise (CV-6) from 1938-41. During World War II he served as Commander Patrol Plane Replacement Squadrons, Patrol Wings, Pacific Fleet; Commander Fleet Air, West Coast; Commander Carrier Division Three; Commander Air Force Pacific Fleet; and Chief of Naval Air Training Command. His final tours, before retirement in 1949, were as Commander Marianas and naval governor of Guam.