The fifth ship to be named Mercury by the Navy, AK-42 was launched as SS Lightning by Federal Shipbuilding & Dry Dock Go., Kearny, New Jersey, 15 July 1939; sponsored by Mrs. Royal S. Copeland; purchased on the ways by Moore McCormack Lines and placed in operation by them as SS Mormactern 30 October 1940; acquired by the Navy 20 June 1941; converted, and commissioned as Mercury 1 July 1942.

The converted merchantman completed her maiden wartime voyage at San Francisco, California, on 27 September 1942 having carried a mixed cargo, which included drummed petroleum products, landing mats, dynamite, engineering equipment, and food, to Noumea and Guadalcanal.

Over the next 15 months she continued to carry essential materiel to the South Pacific, supporting Allied forces as they pushed through the Solomons and the Gilberts.

Arriving at San Francisco 17 March, Mercury underwent overhaul and then resumed cargo runs between Pearl Harbor and the U.S. West Coast and among the Hawaiian Islands.

Arriving in Lingayen Gulf 9 January 1945, she unloaded in 2 days and returned to Leyte to embark units of the 38th Infantry Division, assigned to cut off a Japanese retreat into the Bataan Peninsula.

Completing the mission off the coast of Zambales Province 29 January, she steamed for Ulithi, joined ServRon 10, and underwent alterations to enable her to transfer cargo at sea.

Arriving at Norfolk, Virginia, 16 October 1958, she completed her last mission in that capacity, a 10-month tour during which she set records for underway transfer rates while supporting the 6th Fleet during the Lebanon crisis.