Gar's maiden patrol, from 2 February to 28 March 1942, was conducted around Nagoya and the Kii Channel entrance to the Inland Sea of Japan.

During her second war patrol, from 19 April to 8 June, she fired on a freighter off Kwajalein atoll, which her commanding officer believed was hit, but the ship did not sink.

Her fourth war patrol, from 17 September to 7 November, took her to the northernmost waters in the Gulf of Siam, where on 19 October she laid 32 mines in the entrances to Bangkok.

)[13] Her sixth, from 9 February to 2 April, brought numerous contacts with targets which could not be closed to firing range because of vigilant enemy aircraft and antisubmarine patrol ships.

As commanding officer of USS Gridley (DD-380), his first award was for assisting in the rescue of RM1/c George R. Tweed from the Japanese-held island of Guam.

)[14] En route from Fremantle to Pearl Harbor on her ninth war patrol, from 8 August to 13 September, Gar scouted off Timor and scored hits on a freighter in Makassar Strait.

Gar returned to Pearl Harbor 30 November 1943, now in the hands of George W. Lautrup, Jr. (class of 1934),[16] to resume combat duty in the Pacific, based out of Fremantle.

[17] Her 11th war patrol, from 3 March to 21 April, found her performing lifeguard duty for aviators making the first carrier-based air strikes on Palau.

Her 12th patrol, from 20 May to 5 July, was spent in the Bonin Islands area, where she made gunfire attacks on a convoy of Japanese sea trucks, leaving a small freighter raging in flames and dead in the water.

On 27 November 1944, an Allied PBY Catalina mistook her for a Japanese submarine and attacked her in the Celebes Sea 20 nautical miles (37 km; 23 mi) southeast of the Sibutu Passage at 04°33′N 119°50′E / 4.550°N 119.833°E / 4.550; 119.833.

On her 15th and final war patrol, from 4 to 27 December, she landed 35 tons of supplies on the west coast of Luzon, near Darigayos Inlet[19]: 171–173  on 11 December, returning to Pearl Harbor with urgent intelligence documents including maps locating enemy gun emplacements, beach defenses, troop concentrations, and fuel and ammunition dumps on Luzon.

[21]: 255 After overhaul in the Pearl Harbor Naval Shipyard, Gar put to sea 2 April 1945 to serve the remainder of the war as a target trainer for antisubmarine ships at Saipan and Guam, Marianas Islands.

She departed Apra Harbor, Guam, on 7 August 1945, proceeding via Hawaii, San Francisco, California, and the Panama Canal to the Portsmouth Naval Shipyard in Kittery, Maine, where she arrived 20 October.