Jun'yō's keel was laid down by Mitsubishi on Slipway No.
3 at their shipyard in Nagasaki on 20 March 1939.
She was yard number 900 and had the name Kashiwara Maru at that time.
The ship was purchased on 10 February 1941 by the Navy Ministry and she was temporarily referred to as No.
1001 Ship (Dai 1001 bankan) to keep her conversion secret.
She was launched on 26 June 1941 and commissioned on 3 May 1942 as Jun'yō.

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.
She was tasked to support Operation AL, an attack planned to seize several Aleutian Islands to provide advance warning in case of an American attack from the Aleutians down the Kurile Islands while the main body of the American fleet was occupied defending Midway.
Jun'yō carried 18 A6M2 Zeros and 18 D3As for this operation.
At dawn on 3 June, she launched 9 Zeros and a dozen D3As to attack Dutch Harbor on Unalaska Island.
They had to turn back due to bad weather, although an American PBY Catalina reconnaissance aircraft was shot down by a Zero.
A second airstrike was launched later in the day to attack a group of destroyers discovered by aircraft from the first attack, but they failed to find the targets.
Another airstrike was launched on the following day by the two carriers that consisted of 15 Zeros, 11 D3As, and 6 B5Ns and successfully bombed Dutch Harbor.
As the aircraft from Jun'yō were regrouping after the attack, they were attacked by 8 Curtiss P-40 fighters that shot down 2 Zeros and a pair of D3As while losing 2 of their own.
Another D3A got lost and failed to make it back to the carrier.
Shortly after the aircraft were launched, the Americans attacked the carriers, but failed to inflict any damage.
A Martin B-26 Marauder bomber and a PBY were shot down by Zeros, and a Boeing B-17 Flying Fortress bomber was shot down by flak during the attack.

Jun'yō had initially been designated as an auxiliary aircraft carrier (Tokusetsu kokubokan), but following the loss of four Japanese fleet carriers in the Battle of Midway, she was redesignated as a regular carrier (Kokubokan) in July.
Captain Okada Tametsugu assumed command on 20 July 1942.
Upon arrival at Truk on 9 October, together with her sister ship Hiyō, the ship was assigned to the Second Carrier Division to begin operations against American forces in the Guadalcanal area as part of the 3rd Fleet.

On the night of 16 October, the two carriers were ordered to attack the American transports off Lunga Point, Guadalcanal and they moved south to their launching point 180 miles (290 km) north of Lunga.
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.
Jun'yō's eight B5Ns engaged Lardner and also failed to hit their target, not least because they were attacked by Marine Grumman F4F Wildcat fighters at 07:32.
The Marine pilots shot down three B5Ns on their first pass and severely damaged another pair which were also forced to crash land.
They then shot down the three remaining bombers after they missed Lardner.
The defending Zeros were only able to shoot down one Wildcat at the cost of one of Hiyō's fighters forced to crash land, although they claimed to have shot down thirteen Marine fighters.
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.

In late October 1942, during the Guadalcanal Campaign, Jun'yō took part in the Battle of the Santa Cruz Islands.
At this time, her air group consisted of e
Jun'yō's keel was laid down by Mitsubishi on Slipway No. 3 at their shipyard in Nagasaki on 20 March 1939. She was yard number 900 and had the name Kashiwara Maru at that time. The ship was purchased on 10 February 1941 by the Navy Ministry and she was temporarily referred to as No. 1001 Ship (Dai 1001 bankan) to keep her conversion secret. She was launched on 26 June 1941 and commissioned on 3 May 1942 as Jun'yō.
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. She was tasked to support Operation AL, an attack planned to seize several Aleutian Islands to provide advance warning in case of an American attack from the Aleutians down the Kurile Islands while the main body of the American fleet was occupied defending Midway. Jun'yō carried 18 A6M2 Zeros and 18 D3As for this operation. At dawn on 3 June, she launched 9 Zeros and a dozen D3As to attack Dutch Harbor on Unalaska Island. They had to turn back due to bad weather, although an American PBY Catalina reconnaissance aircraft was shot down by a Zero. A second airstrike was launched later in the day to attack a group of destroyers discovered by aircraft from the first attack, but they failed to find the targets. Another airstrike was launched on the following day by the two carriers that consisted of 15 Zeros, 11 D3As, and 6 B5Ns and successfully bombed Dutch Harbor. As the aircraft from Jun'yō were regrouping after the attack, they were attacked by 8 Curtiss P-40 fighters that shot down 2 Zeros and a pair of D3As while losing 2 of their own. Another D3A got lost and failed to make it back to the carrier. Shortly after the aircraft were launched, the Americans attacked the carriers, but failed to inflict any damage. A Martin B-26 Marauder bomber and a PBY were shot down by Zeros, and a Boeing B-17 Flying Fortress bomber was shot down by flak during the attack.
Jun'yō had initially been designated as an auxiliary aircraft carrier (Tokusetsu kokubokan), but following the loss of four Japanese fleet carriers in the Battle of Midway, she was redesignated as a regular carrier (Kokubokan) in July. Captain Okada Tametsugu assumed command on 20 July 1942. Upon arrival at Truk on 9 October, together with her sister ship Hiyō, the ship was assigned to the Second Carrier Division to begin operations against American forces in the Guadalcanal area as part of the 3rd Fleet. 
On the night of 16 October, the two carriers were ordered to attack the American transports off Lunga Point, Guadalcanal and they moved south to their launching point 180 miles (290 km) north of Lunga. 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. Jun'yō's eight B5Ns engaged Lardner and also failed to hit their target, not least because they were attacked by Marine Grumman F4F Wildcat fighters at 07:32. The Marine pilots shot down three B5Ns on their first pass and severely damaged another pair which were also forced to crash land. They then shot down the three remaining bombers after they missed Lardner. The defending Zeros were only able to shoot down one Wildcat at the cost of one of Hiyō's fighters forced to crash land, although they claimed to have shot down thirteen Marine fighters. 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.
In late October 1942, during the Guadalcanal Campaign, Jun'yō took part in the Battle of the Santa Cruz Islands. At this time, her air group consisted of e
Japanese aircraft carrier Jun'yō