Dornin entered the United States Naval Academy in 1931. He was an outstanding athlete on the basketball and lacrosse teams, but made his biggest mark in sports as a football end or wide receiver, teaming with Buzz Borries and Slade Cutter to form a powerful "troika". Both his teammates would also distinguish themselves in the coming war, with Cutter also making his mark as a submariner. Cutter would later recall that Dornin "studied all the time"; he graduated 48th out of 442 in the Class of 1935. After serving on the battleship and the destroyer , he graduated from Submarine School. He was a lieutenant aboard the submarine from 1939 to 1941.
Dornin held the rank of lieutenant commander on from 1941 to 1943. After the sneak attack on Pearl Harbor on December 7, 1941, Gudgeon left four days later on the first American submarine war patrol of World War II, with Dornin as the fire control officer. The submarine I-173 became the first enemy warship sunk by an American submarine, courtesy of Gudgeon. Dornin eventually became the executive officer. He was given command of in 1943. In September of that year, he left on his first patrol in Trigger, prowling the East China Sea north of Formosa. On September 17, Trigger sank the Japanese freighter Yowa Maru. Four days later, the tankers Shiriya and Shoyo Maru and the freighter Argun Maru fell victim to the submarine. Though faulty torpedoes had prevented an even higher score, Trigger concluded a very successful patrol, returning to Midway Island on September 30. For Dornin's next patrol, he departed from Midway on October 22, bound for the East China and Yellow Seas. On November 1, Trigger scored hits on two freighters, one of which was seen to sink, before being forced to dive to evade depth-charge attacks by Japanese escort ships. The next day, the freighters Yawata Maru and Delagoa Maru were sent to the bottom several hours apart. On November 13, Trigger located a convoy and sank the largest ship, believed to be a transport. The last victim of the patrol was the freighter Eizan Maru on November 21. Trigger reached Pearl Harbor on December 8. The third patrol began on January 1, 1944, targeting the Truk-Guam shipping lanes. On January 27, Trigger encountered an enemy Ro-class submarine. Neither managed to inflict any damage on the other before contact was lost. Four days later, Trigger sank the coastal minelayer Nasami and the converted submarine tenderYasukuni Maru. The patrol concluded with a return to Pearl Harbor on February 23. USS Trigger was awarded the Presidential Unit Citation for her fifth, sixth and seventh patrols. Dornin was in command of the sixth and seventh. With initial reluctance, after nine patrols, Commander Dornin became an aide to Chief of Naval Operations Admiral Ernest King, beginning in 1944; he accompanied King to the Yalta and Potsdam Conferences in 1945. The Joint Army–Navy Assessment Committee credited him with ten ships sunk in three patrols in command of Trigger, putting him in a tie among American submarine commanders for 15th-21st by number of ships; his first Trigger patrol was also rated the 16th most successful of the war by tonnage.
Post-war
After Admiral King retired in December 1945, Dornin became Fleet Admiral Chester W. Nimitz's aide. Post-war postings include:
