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.

She sailed 24 October with the Southern Attack Group of Admiral H. Kent Hewitt's Western Naval Task Force, which gathered at midnight 7–8 November off the Moroccan coast.

Procyon participated in the assault operations which brought about the surrender of Cape Fedhela and Casablanca and returned to Norfolk 30 November for conversion to an attack cargo ship.

Unloading army vehicles at Naples, following the invasion at Salerno, Procyon moved to Arzew, Algeria and commenced a program of shipboard indoctrination and practice assault landings for the training of Free French Forces, including Algerian and Senegalese troops.

She entered the Clyde River 9 December and docked at Glasgow to off-load mail, army personnel, and much needed aircraft parts and ammunition cases.

Ten days later she passed out to sea in convoy and under escort by HMS Searcher, bound for the U.S. She arrived Norfolk 2 January 1944, and following independent operations, shifted to New York 14 February to load cargo for shipment to the British Isles.

She operated in support of the Okinawa invasion during April, returning to San Francisco 19 May to begin a series of shuttle runs to Hawaii, Ulithi and Samar.