Clearing Norfolk, Virginia, 6 May 1941, Castor arrived at San Diego, California, 20 May to begin a series of cargo voyages to Pearl Harbor.

From January 1944, her voyages from the west coast were to bases in the Marshall Islands, and after a brief overhaul at Seattle, Washington, Castor reported at Manus 18 September for duty with famed task force TF 58.

Operating primarily from Manus and Ulithi, she replenished the fast carrier task force at sea thus helping to expedite the smashing series of raids which pushed the Japanese ever westward.

Between December 1945 and February 1947, Castor supplied occupation forces in the Far East, calling at Guam, Saipan, Qingdao, Hong Kong, and Japanese ports.

Leaving Yokosuka astern 18 March 1952, Castor ran into a fierce storm which damaged her engines and left her dead in the water for 22 hours until she could be towed back to Japan for emergency repairs.

While bound for Subic Bay on 19 October 1956, Castor fought her way unscathed through a furious typhoon, and next day received a distress message from the Philippine merchantman SS Lepus.