She was laid down as the passenger liner Kashiwara Maru (橿原丸), but was purchased by the IJN in 1941 while still under construction and converted into an aircraft carrier.

The ship was ordered in late 1938 as the fast luxury passenger liner Kashiwara Maru by Nippon Yusen Kaisha (the Japan Mail Steamship Company).

In exchange for a 60 percent subsidy of her building costs by the Navy Ministry, she was designed to be converted to an auxiliary aircraft carrier, one of 10 such ships subsidized by the IJN.

[8] The ship's primary armament consisted of a dozen 40-caliber 12.7 cm (5.0 in) Type 89 anti-aircraft (AA) guns in six twin-gun mounts on sponsons along the sides of the hull.

Jun'yō was also initially equipped with eight triple mounts for 25 mm Type 96 light AA guns alongside the flight deck.

The first of these was mounted on the top of the island in July 1942, shortly after she was completed, and the other was added later in the year on the port side of the hull, outboard of the rear elevator.

[12] Upon commissioning, the ship was assigned to the Fourth Carrier Division of the 1st Air Fleet, together with Ryūjō, under the command of Rear Admiral Kakuji Kakuta.

They had to turn back due to bad weather, although an American PBY Catalina reconnaissance aircraft was shot down by a Zero.

At 05:15 each ship launched nine each A6M Zeros and B5Ns (one of Jun'yō's B5Ns was forced to turn back with mechanical problems) which reached the target and discovered two destroyers bombarding Japanese supply dumps on Guadalcanal around 07:20.

Hiyō's aircraft attacked Aaron Ward seven minutes later without effect, although the American ship shot down one B5N and damaged another which was forced to make a crash landing.

[14][15] Hiyō was forced to depart the area after a fire reduced her top speed to 16 knots (30 km/h; 18 mph) and transferred three Zeros, one D3A and five B5Ns before she left.

Rear Admiral Kakuji Kakuta, commander of the Second Carrier Division, also transferred and hoisted his flag aboard Jun'yō.

At 05:00 on 26 October 1942, she had launched fourteen Zeros and a few D3As to land at Henderson Field on Guadalcanal, which had been falsely reported by the Imperial Japanese Army as in their hands, but they were greeted by Marine Grumman F4F Wildcats and all were shot down.

At 09:30, Jun'yō launched another air strike that attacked the carrier Enterprise, the battleship South Dakota and the light cruiser San Juan, scoring hits on the latter two, but inflicting little substantial damage.

Shortly afterward, more aircraft were launched to attack the American ships, including six B5Ns and six D3As escorted by half a dozen Zeros.

The Americans finally ordered the ship abandoned and the last wave of dive bombers hit Hornet twice more, but inflicted little further damage.

Six of her Zeros were on Combat Air Patrol when the convoy was discovered by two SBDs from Enterprise and shot down one dive bomber after it had made its spot report.

That afternoon, Enterprise had been discovered by a searching B5N, and Jun'yō launched an air strike with her remaining aircraft, but they failed to locate the American carrier.

[14] Her air group was detached to Rabaul on 2 April to participate in Operation I-Go, a land-based aerial offensive against Allied bases in the Solomon Islands and New Guinea.

[14] The ship's air group was deployed to Buin, Papua New Guinea, on 2 July in response to the American attack on Rendova Island on 30 June.

En route from Truk to Kure on 5 November 1943, Jun'yō was hit off Bungo Suidō by a torpedo from the submarine Halibut.

[22] The new base was closer to the oil wells in Borneo on which the Navy relied and also to the Palau and western Caroline Islands, where the Japanese expected the next American attack.

A pair of Zeros and 6 D4Ys bound for Rota spotted the carriers Wasp and Bunker Hill en route, but failed to inflict any damage on the American ships while losing 5 D4Ys to anti-aircraft fire.

They discovered the retiring Japanese fleet during the afternoon of the following day and Vice Admiral Marc Mitscher ordered an air strike launched.

[27] After repairs at Kure, Jun'yō remained in the Inland Sea without aircraft until 27 October, when she was tasked to transport material to Borneo.

On 3 November, she was attacked by the submarine Pintado near Makung, but her escorting destroyer, Akikaze, deliberately intercepted the torpedoes and sank with no survivors.

Having loaded 200 survivors of the battleship Musashi, Jun'yō was attacked by the submarines Sea Devil, Plaice and Redfish early in the morning of 9 December 1944.

[14] The repairs were abandoned in March 1945 for lack of materials and the ship was moved from the dock to Ebisu Bay, Sasebo, on 1 April.