Half Moon spent her first months in shakedown training off California and was then assigned to the Pacific Fleet. Departing San Diego 25 August 1943, she embarked a Marine air group at Pearl Harbor and steamed into Vila Harbor, New Hebrides, 14 September. The tender then sailed to Brisbane, Australia, and thence to Namoai Bay, on Sariba Island, New Guinea, arriving 6 October. At Namoai Bay Half Moon began her tending duties. Her embarked squadron, flying PBY Catalina aircraft, conducted night antishipping strikes in the New Guinea area. With the able support of Half Moon and her sister tenders these missions - Black Cat strikes - achieved important results in the destruction of Japanese transports.

Half Moon departed for Brisbane 21 December 1943, remained until 10 February 1944, and then steamed into a succession of New Guinea ports on the way to her new operating base, Finshafen. There she resumed her support of seaplane operations in the New Guinea theater.

After tending seaplanes on rescue missions from Humboldt Bay in May, Half Moon spent several months pinch-hitting for transports in the Pacific area,stopping at Brisbane, Manus, Milne Bay, and other ports. She took up Black Cat operations again on 25 August 1944 from Middleburg and later Morotai. Steaming out of Morotai 6 October, Half Moon joined a small convoy en route to Leyte Gulf to assist in the developing operations for the recapture of the Philippines. They arrived Leyte Gulf 21 October and Half Moon immediately steamed down the eastern coast of Leyte in search of a proper anchorage for her seaplane operations.

Anchoring in Hinamangan Bay, Half Moon came under air attack 23 October, and soon realized that her anchorage was a rendezvous point for Japanese planes attacking Leyte. Late 24 October Half Moon's radar began to pick up two large surface units converging and it was soon clear that she was to be a witness to the last engagement between battle lines of surface ships - the Battle of Surigao Strait. The tender cautiously slipped out from behind Cabugan Grande Island and was given permission to proceed up the coast of Leyte between, but well to the west of the two fleets. She watched the spectacle of Admiral Olendorf's big guns pounding the Japanese ships, and after the battle returned to Hinamangan Bay. Another fierce air attack, however, soon convinced the captain that San Pedro Bay, further north, offered a more hospitable base for operations.

Half Moon weathered two severe storms, 29 October and 8 November 1944, and operated with her seaplanes in Leyte Gulf until 27 December. She was then designated as part of the support convoy for the Mindoro Landing, and departed for Mangarin Bay 27 December. The convoy, known as Uncle plus 15, encountered some of the most prolonged and determined air attacks of the war as the Japanese strove mightily to prevent reinforcements at Mindoro. Air cover provided by land-based aircraft stopped only some of the attackers. Suicide planes, bombs, and strafing hit many ships. Liberty Ship John Burke, loaded with ammunition, exploded leaving vitually no trace after a kamikaze hit, tanker Porcupine and tender Orestes were severely damaged, and other ships also suffered hits.

Nevertheless, the convoy drove through giving the Japanese planes a hot time with the concentrated AA fire. During these 3 days, gunners on the Half Moon and the other ships were at their stations round the clock, Half Moon accounting for at least two and possibly four of the attacking aircraft.

The convoy arrived at Mindoro 30 December, air attacks continued. On 4 January during one of these attacks a large bomb skipped over Half Moon's fantail, failing to explode. The tender remained in Mangarin Bay tending seaplanes until returning to Leyte Gulf 17 February.

Following the victorious Philippine invasion, Half Moon sailed to Manus and Humboldt Bay. On 30 May, she got underway for the Philippines again, arriving Tawi Tawi, Sulu Archipelago, 11 June. She supported seaplane anti-submarine searchers from Tawi Tawi Bay until early August, and then carried out the same mission from Mangarin Bay, Mindoro.

Following Japan's surrender, Half Moon proceeded to Subic Bay, Phiilippines, and from there got underway for Okinawa 30 August. The afternoon of the next day signs of a storm were evident and by 1 September Half Moon was engulfed in a raging typhoon, with winds up to 120 knots and barometer readings of 27.32. Smart seamanship allowed her to weather the storm, and she arrived safely at Okinawa 4 September.

Half Moon departed Okinawa for Manila 1 October, operated in that area for about a month, and departed 7 November for deactivation. She arrived Seattle 1 December 1945, steamed to San Diego 12 April 1946, and decommissioned there 4 September 1946. Placed in reserve, she was taken out, refitted, and loaned to the Coast Guard in September, 1948.Half Moon served through the 1960s as a weather ship for the Coast Guard. Based at Staten Island, N. Y., Half Moon's crew collected hourly weather data at a number of ocean stations in the Atlantic Ocean and acted as an emergency Search and Rescue vessel to assist both planes and ships in distress.

Half Moon received two battle stars for her World War II service.