On 7 December 1941, "I-122" laid forty-two Type 88 Mark 1 mines northeast of Singapore, then took up her patrol station off the eastern entrance of Johore Strait along with "I-121". In East Asia, World War II began on 8 December 1941, and Japanese forces invaded British Malaya to the north of Singapore that day. Reassigned to the Southern Submarine Force on 12 December 1941, she called at Cam Ranh Bay in Japanese-occupied French Indochina on 14 December 1941. On 18 December 1941, she got back underway to support the Japanese invasion of the Philippines, bound for the vicinity of Davao on Mindanao in company with "I-124". Reassigned to Submarine Group "A" along with "I-121", "I-123", and "I-124" on 26 December 1941, she concluded her patrol on 31 December 1941 when she and "I-124" arrival at newly captured Davao where "Chōgei" serviced "I-121", "I-122", and "I-124".