Assigned to the Pacific Fleet, Monongahela departed Norfolk in November 1942 for Aruba, Netherlands West Indies, where she loaded oil and then steamed to Nouméa, New Caledonia, via the Panama Canal to supply American forces engaged in the struggle for Guadalcanal.

In January 1944, following a stateside overhaul and a Pearl Harbor fuel run, the big auxiliary joined the 5th Fleet support group for the invasion of the Marshall Islands, supplying various ships and small bases at Kwajalein, Eniwetok, and Majuro as those atolls were captured during the campaign.

The ship also supported the invasions of Iwo Jima and Okinawa, narrowly escaping damage from suicide air attacks at Kerama Retto, on 16 April 1945.

At the end of the Okinawa operation in June, Monongahela steamed to San Francisco for a much needed overhaul which was still in progress when hostilities ceased on 15 August.

[citation needed] The ship once again recommissioned as part of MSTS on 28 December 1956, operating along the Atlantic coast for eight months and then decommissioning on 22 August 1957 to enter the Maritime Administration's Reserve Fleet, at Beaumont, Texas.