During the Japanese invasion of Malaya, Choy served as a second lieutenant in the women's auxiliary arm of the Singapore Volunteer Corps, where she acquired the nickname "Gunner Choy". She was also a volunteer nurse with the Medical Auxiliary Service. After the fall of Singapore in 1942, the Choys set up a canteen at the Tan Tock Seng Hospital, after all the patients and doctors had been moved from the Miyako Hospital (former Woodbridge Hospital), where they soon started a regular ambulance run for British civilian internees. The couple helped prisoners-of-war interned in Changi Prison by passing to them cash and parcels containing fresh clothing, medicine and letters during their deliveries. They incurred further risk by sending in radio parts for hidden receivers until the Japanese crackdown following Operation Jaywick.