After shakedown, "Procyon" was assigned to the Naval Transportation Service and she reported to Norfolk, Va. on 25 August to load cargo for Caribbean bases. She sailed 2 September reaching as far as Panama before returning to Norfolk on the 23rd. She put to sea again on 10 October, transited the Panama Canal 20 October bound for San Pedro, Calif., and proceeded to Pearl Harbor 12 November. "Procyon" remained at Pearl 18–24 November and returned to California at the end of the month. She was entering the Mare Island Navy Yard the day of the attack on Pearl Harbor, when, receiving emergency sailing orders, she shifted to a San Francisco dock to load a cargo of blood plasma and medical supplies. Departing San Francisco 12 December, she entered Pearl Harbor on the 19th. "Procyon" returned to San Francisco 6 January 1942 and commenced round-the-clock loading of fighter planes and their Marine pilots destined for Pago Pago, Samoa. She completed three round trips between those points by 23 April.