Following the outbreak of World War II in Europe, she sailed to the United States where she operated under charter from the Maritime Commission as Rita Maersk and later as Larwin.

After completing two cruises, she was laid up at Boston, Massachusetts, until 18 September 1941 when she was acquired by the United States Navy from the Maritime Commission.

Following an abbreviated shakedown, Pegasus loaded military cargo including 600 depth charges and sailed in convoy for Iceland on 27 December.

There, raging winter storms driven by winds in excess of 100 knots (190 km/h; 120 mph) imperiled the ship and her cargo, and she did not return to the U.S. East Coast until late February.

On 24 March Pegasus joined her second Iceland-bound convoy, and during the spring and summer months of 1942 she completed three round trips to Iceland and back.

Pegasus returned to Boston, Massachusetts, on 11 October; and, after completing repairs to her main engine, she sailed on 16 November to begin extended operations in the Caribbean.

Between 18 January 1943 and mid-May she made four runs to Puerto Rico and back; thence, during June and July she hauled U.S. Navy cargo to Cristobal, Panama Canal Zone.

During the next ten months she hauled thousands of tons of war material to American bases in Cuba, Puerto Rico, and the Virgin Islands.