Portunus was laid down by the Federal Shipbuilding and Drydock Co., Kearny, New Jersey, as LSM–275 1 August 1944; launched 11 September 1944; and commissioned 6 October 1944.

[1] After shakedown off Norfolk, Virginia, LSM–275 steamed for Key West, Florida, the Panama Canal, and San Diego, California, before reaching Pearl Harbor on 28 January 1945.

[1] After a trip to Guam, she steamed from Pearl Harbor in task unit TU 51.13.4 via Eniwetok and Saipan to Okinawa to land troops and supplies on the "last stepping stone" to Japan.

Departing 28 June, she then put into Tulagi, the Russells, Eniwetok, Guam, and Saipan, before reaching Pearl Harbor 19 August, 5 days after the war's end.

During this time, she continued to base at the New York Shipyard, Brooklyn, and to periodically pick up cable at Newington, New Hampshire.