In 1944, after shakedown in the Caribbean, Haynsworth departed New York 20 September escorting Queen Mary with Prime Minister Winston Churchill on board. Rendezvousing with British escorts, she returned to New York and sailed 26 September via the Canal Zone and San Pedro, arriving Pearl Harbor 20 October. Haynsworth sailed 16 December for Ulithi and joined Vice Admiral J. S. McCain's Fast Carrier Task Force 38 for the final assaults on the Japanese. During the next 3 months she operated with the 3d and 5th Fleets as part of the screen for the Fast Carrier Task Force; the primary mission being to conduct air strikes against strategic Japanese positions along the China coast, and Formosa, and to harass enemy shipping during the landings at Luzon 9 January 1945.

The day after the invasion was launched, Task Force 38 moved into the South China Sea and conducted raids on the China coast and Indochina, doing much damage to the enemy. Launching one final raid against Okinawa, Haynsworth retired to Ulithi 26 January. She sortied 10 February with Admiral Marc Mitscher's Fast Carrier Force 58 for strikes against airfields, factories, and shipping in the Tokyo area. Heavy fighter sweeps were launched 16 February to cover the airfields around Tokyo Bay. Despite heavy weather with low ceiling, most of the target areas were effectively neutralized. During the afternoon three Japanese picket boats that had evaded detection in thick fog were spotted by Haynsworth and promptly sunk, taking 12 prisoners. In addition to damaging aircraft frame and engine plants, a number of ships and small craft were attacked and sunk in Tokyo Bay, the biggest prize being the 10,600-ton Yamashro Maru.

As the Pacific war approached its climax, Haynsworth again sailed from Ulithi for further strikes against Japan. Massive air attacks were launched against airfields on Kyushu and ships in the Inland Sea 18 and 19 March, indicting heavy damage on the dwindling Japanese air and sea power. After participating in the bombardment of enemy shore positions on Minami Daito Shima 28 March, she sailed for Okinawa. Landings were made on the Japanese fortress April, with Task Force 58 providing support, and Haynsworth frequently aiding in the destruction of enemy aircraft during the many attacks,, when "the fleet had come to stay." Only after she was crashed by a kamikaze 6 April 1945 did she have to retire to Mare Island via Ulithi for repairs.

After repairs Haynsworth had duty at Treasure Island, Calif., as a training ship from 17 July to 5 September. After several months of operations at Pearl Harbor, she sailed for the east coast 14 January 1946, reaching Boston 26 April for a year in the Reserve Fleet. Returning to active service in March 1947, Haynsworth based her operations from Algiers, La., conducting reserve training cruises in the Gulf and in the Caribbean until the summer of 1949.

Haynsworth sailed 6 September 1949 for her first duty with the 6th Fleet in the Mediterranean, returning to Norfolk 7 February 1950. She arrived Charleston 10 days later, decommissioned there 19 May and joined the Reserve Fleet.