On completion, she was assigned directly to the Combined Fleet under the command of Captain Tomeo Kaku and was dispatched to the front lines in the Second Sino-Japanese War paired with the seaplane tender Kamoi.

Afterwards, she was reassigned back to the Combined Fleet and began training operations on the use of her midget submarines through September, developing tactics for attacking other vessels (using Chitose as a target) and penetrating enemy naval bases.

At the time of the attack on Pearl Harbor, Chiyoda was docked at Kure and continued training operations to 20 March 1942, when she was assigned to Vice Admiral Teruhisa Komatsu’s 6th Fleet together with the submarine tender Nisshin and Aikoku Maru.

The operation was cancelled on the loss of the Japanese aircraft carriers during the Battle of Midway, and Chiyoda returned to Hashirajima with her submarines on 14 June without having seen combat.

[2] After conversion, Chiyoda was assigned to the 3rd Fleet and departed Yokosuka for Saipan, Guam and Palau, Balikpapan and Davao on 1 March as part of emergency reinforcements following the fall of Kwajalein to the US, returning to Kure on 10 April.

During the Battle of the Philippine Sea on 19 June, she was part of the Van Force with carriers Chitose, Zuihō, battleships Yamato, Musashi, Kongō, Haruna and cruisers Atago, Takao, Maya and Chōkai.

[3] On 20 October 1944 Chiyoda departed Oita as part of Admiral Jisaburō Ozawa’s Decoy Force intended to lure the American fleet away from the landing beaches in the Philippines in the Battle of Leyte Gulf.

This force included Zuikaku, Zuihō, Chitose and Chiyoda, and the hybrid battleship-carriers Hyūga and Ise, though the six carriers were divested of all but 108 aircraft, accompanied by cruisers Oyodo, Tama, Isuzu.

[4] On 25 October, both Chiyoda and Chitose were sunk by a combination of naval bombers, cruiser shellfire and destroyer-launched torpedoes during the Battle off Cape Engaño.

[3] She was finished off by gunfire from four cruisers, USS Santa Fe, Mobile, Wichita and New Orleans,[5] along with nine destroyers, all under the command of Rear Admiral Laurence DuBose.