She then loaded a complement of 31 landing craft and a Beach Party unit before sailing to the West Coast where she became the flagship of Transport Division (TransDiv) 26.

She spent most of December 1943 conducting landing exercises off San Clemente, California, with elements of the 25th Regimental Combat Team, 4th Marine Division.

As a component of Task Group (TG) 54.4, Admiral R. K. Turner's Northern Attack Force for the invasion of Saipan and Tinian, the transport again loaded elements of the 4th Marine Division and sailed on 29 May.

Covered by an intensive air-sea bombardment, and receiving incoming fire from enemy artillery, mortars, and automatic weapons, the assault wave of marines landed at 0843.

Sumter arrived at Pearl Harbor on 21 July and trained there until 12 August when she was routed to Guadalcanal for additional amphibious exercises with the 81st Infantry Division.

At the latter port, she loaded troops of the 6th Army Division and sailed with the San Fabian Attack Force on 30 December 1944 for the Lingayen Gulf area of the Philippines.

Sumter arrived at Guadalcanal on 19 February 1945 and began amphibious exercises with the 22d Regimental Combat Team of the 6th Marine Division in preparation for the invasion of Okinawa.

After landing 1,352 marines of the assault waves, the transport remained off the beach until sailing for the United States, via the Mariana Islands and Pearl Harbor, on 5 April.

Sumter arrived at Seattle, Washington, on 22 November 1945 and remained there until 25 January 1946 when she moved to San Pedro, California, to unload her landing craft.

After being sold by the Navy, Sumter was converted into the first container ship, the SS Gateway City[1] at Mobile SR Inc., of Chickasaw, Alabama in September 1957.