Canopus began her regular schedule of services in Manila Bay, and each summer based with the fleet at Tsingtao, China, with occasional training cruises to various Chinese and Japanese ports, and to the British and French colonies.

On 7 December 1941, Canopus, aging but able, lay at Cavite Navy Yard, as tender to Submarine Squadron 20.

With the Army falling back on Manila, Canopus sailed to Mariveles Bay at the tip of Bataan on Christmas Day.

This time a fragmentation bomb which exploded near the top of the smokestack, resulting in substantial damage to the ship and injuries to 16 men of the gun crews.

The crew continued to care for small craft and equipment of the Army and Navy, or were attached to the improvised naval battalion which fought on Bataan.

The ship's launches were converted into miniature gunboats dubbed Mickey Mouse Battleships, and attacked the Japanese moving south near the shore.

Of the Canopus' crew, 221 were evacuated to Corregidor on 28 February 1942 and served with the Marines' 1st, 2nd, and 3rd Battalions on beach defenses.