On 25 May 1940, she was ordered recommissioned for conversion to a
seaplane tender. Accordingly, she was recommissioned, in ordinary, on 24 June 1940 and moved to the San Francisco yard of the Bethlehem Steel Corporation for conversion. On 2 August 1940,
Thornton was officially redesignated a seaplane tender (destroyer),
AVD-11. Her alterations were completed early in 1941, and she was placed in full commission on 5 March 1941. On 8 April, she reported for duty to the Commander-in-Chief of,
Pacific Fleet, at
San Pedro. Ten days later, the seaplane tender arrived in
Pearl Harbor, and she operated in the
Hawaiian Islands until August 1942. During her 16 months in the islands, she made frequent voyages to
Midway,
Wake Island,
Palmyra and other outlying islands of the 14th Naval District. On the morning of 7 December 1941, she was moored at the Submarine Base at Pearl Harbor. Her action report for that day states that the Japanese opened their
attack on Pearl Harbor at 0756 and that ''Thornton's
crew, led by four reserve ensigns, was at action stations two minutes later. They fought back with every available weapon: four .50-cal. machine guns, three Lewis guns, three Browning automatic rifles, and twelve .30-cal., bolt-action Springfields. The combined fire of Thornton
and accounted for at least one Japanese torpedo bomber and probably discouraged two more from making a run on as the oiler changed berths during the second dive-bombing attack between 0910 and 0917. Thornton'' suffered no casualties during the attack on Pearl Harbor. Following the Pearl Harbor attack, she was stationed at
French Frigate Shoals with as aircraft rescue ships for the planes engaged in the expanded air searches. Coincidentally, the Japanese had planned to use the
French Frigate Shoals as a rendezvous point for the second half of
Operation K, the reconnaissance of Pearl Harbor. The Japanese subsequently cancelled the remainder of
Operation K after the Japanese submarine I-123, sent to scout the area, was forced to depart. After the victory at Midway, she resumed her runs between the outlying islands of the 14th Naval District, though the Japanese occupation had removed Wake Island from her itinerary, until August 1942. On the 25th,
Thornton steamed out of Pearl Harbor, headed north, and arrived at
Kodiak, Alaska, on the 30th. For the next two months, the seaplane tender cruised the icy
Alaskan seas as a part of Task Force 8. She visited Kodiak,
Attu, and
Chernofski before departing Kodiak for Pearl Harbor on 21 October.
Thornton stopped at Pearl Harbor from 30 October to 10 November, then headed for duty in the South Pacific. After short periods of duty at
Suva in the
Fiji Islands,
Funafuti in the
Ellice Islands, and at
Vanikoro in the
Santa Cruz Islands, she moved to
Espiritu Santo in the
New Hebrides, arriving on 18 July 1943. The seaplane tender remained at Espiritu Santo until 11 November, when she put to sea for
Guadalcanal in the Solomons. Between 13 November and 15 November, she made a round-trip run between Guadalcanal and Espiritu Santo to escort from the latter to the former. Following duty in the Solomons and a stop at Pearl Harbor from 5 February to 8 February,
Thornton returned to the west coast at
Mare Island on 17 February 1944. She stayed on the west coast for the next 10 months, conducting routine operations and undergoing extensive repairs. On December 3, 1944, the warship left San Pedro to return to the western Pacific. ==Fate==