After the Japanese attack on Pearl Harbor, Thornhill served with his ship, USS Lexington, during the first month of the war, patrolling the Johnston–Palmyra–Oahu triangle against possible enemy incursions.

In February and March, he participated in the carrier's offensive patrols in the Coral Sea and in the attacks on Japanese installations at Salamaua and Lae located on the northern coast of New Guinea.

During the Battle of the Coral Sea Thornhill piloted one of the 12 Douglas TBD Devastator torpedo bombers launched during mid-morning on 7 May to seek out and destroy Japanese forces converging on the Australian base at Port Moresby.

While VB-2 took some of the fighter pressure off the torpedo bombers, Thornhill and his comrades split formation and attacked the carrier from both directions astern.

Sometime after 11:00, they found the Shokaku, At 11:42, VT-2 commenced its attack; and the carrier began a long, slow turn to the right which allowed each TBD-1 to make its "run without splitting across the stern."

On 9 June 1945, she and the other ships of Escort Division (CortDiv) 55 got underway for Guantánamo Bay and proceeded thence through the Panama Canal to the west coast of the United States.

CortDiv 55 stood out to sea five days later and arrived at Pearl Harbor on the 19th to join the Destroyer Force, Pacific Fleet.

Thornhill and her division departed with RMS Empress of Australia on 8 August, bound for the Marshalls, and reached Eniwetok the day after hostilities with Japan ceased.

The destroyer escort sailed for home on 2 February and, after calling at San Diego, arrived at the Boston Navy Yard on 7 March.

Thornhill was transferred to Italy under the Military Assistance Program on 10 January 1951 and was struck from the Navy List on 26 March that same year.