The ship was laid down on 29 June 1942 by Bethlehem Steel Co., Sparrows Point, Maryland, as a type T3-S-A1 tanker named SS Citadel, under a Maritime Commission contract (MC hull 520).

In March 1944 she was altered at Norfolk for Pacific service, and sailed for the Panama Canal, Pearl Harbor, and Kwajalein where she fueled transports bound for the Marianas invasions begun in June 1944.

She sailed 26 March from Ulithi for Okinawa, and after refueling 5th Fleet striking units at sea, closed Hagushi Beach on 5 April to fuel radar picket destroyers.

Niobrara entered Tokyo Bay on 30 August to witness the surrender, and after three months' occupation duty, sailed for Guam, where she aided in salvage work.

She lay in reserve at Philadelphia, Pennsylvania, until recommissioning on 5 February 1951 for three years' service with the Atlantic Fleet along the east coast, in the Caribbean, and in the Mediterranean.