The vessel returned to New York City on 5 September to take on a cargo of aviation gasoline and diesel oil and got underway in a convoy bound for Guantánamo Bay, Cuba.

Ammonusuc arrived at Pearl Harbor on 6 November and, shortly thereafter, made a voyage to Johnston Island to carry several thousand barrels of aviation gasoline to facilities ashore there.

During the next seven months, she shuttled between Saipan and Iwo Jima delivering fuel oil and aviation gasoline in support of U.S. Army Air Force operations on that island.

Following the end of the fighting in mid-August, Ammonusuc carried medical supplies and other stores to working parties at Chichi Jima.

Upon the completion of dock trials, she departed San Francisco Bay on 17 April in a convoy bound for the Panama Canal Zone.