The US Sixth Army was activated in January 1943, commanded by Lieutenant General Walter Krueger. It assumed control of the US Army units undertaking Operation Cartwheel, the campaign to isolate the neutralise the Japanese base at Rabaul in New Britain. Following the completion of Cartwheel, Sixth Army continued moving up the coast of New Guinea. Similar in conception to the island hopping operations of the central Pacific, the object of the attacks was to land, establish a garrison and airfield which could support the next strike, and then move on.

In September 1944, Sixth Army was released from operations in New Guinea by Eighth Army. On 20 October 1944, X Corps and XXIV Corps under Sixth Army invaded Leyte in the Philippines. By December, Leyte was almost secure, and Sixth Army was relieved again by Sixth Army to prepare for the invasion of Luzon. As a prelude to that invasion, the island of Mindoro was invaded by the Western Visayan Task Force comprising the 19th and 503rd Regimental Combat Teams. Luzon itself was invaded on 9 January 1945 by I and XIV Corps. Sixth Army units fought south until they met up those of Eighth Army advancing from around Manila. Sixth Army then continued to clear the north of Luzon until the end of the war.

Sixth Army was to have provided the ground forces for the first phase of the invasion of Japan, but the surrender changed that. Occupation duty then followed for a short while until Sixth Army returned to the United States. Sixth Army then took responsibility for training of Army forces from part of the continental United States until it was inactivated as part of force reductions.