[3] Naval architect Vice Admiral Yuzuru Hiraga was able to keep the design from becoming dangerously top-heavy in its early years by continually rejecting demands from the Imperial Japanese Navy General Staff for additional equipment to the upper decks.

[4] The Myōkō class displaced 13,500 t (13,300 long tons), with a hull design based on an enlarged version of the Aoba-class cruiser.

[4] Propulsion was by 12 Kampon boilers driving four sets of single-impulse, geared-turbine engines, with four shafts turning three-bladed propellers.

[4] Myōkō’s main battery was ten 20 cm/50 3rd Year Type naval guns, the heaviest armament of any heavy cruiser in the world at the time, mounted in five twin turrets.

[4] Myōkō was laid down at the Yokosuka Naval Arsenal on 25 October 1924 and launched and named on 16 April 1927 in a ceremony attended by Emperor Hirohito, and was commissioned into the Imperial Japanese Navy on 31 July 1929.

During a naval review off Kobe on 26 October 1930, stack gases caused problems on the bridge, resulting in a lengthening of the forward smokestack by 2.0 m.[4] During the First Shanghai Incident of February 1932, the cruisers escorted the transports conveying elements of the Imperial Japanese Army (IJA) to the continent.

A second reconstruction and retrofit was completed in April 1941, doubling the number of torpedoes to 16, adding another eight 25-mm antiaircraft guns, and bulges to the hull to improve stability.

Sentai-5 was commanded by Rear Admiral Takeo Takagi and deployed from Palau cover for the landings of Japanese forces under "Operation M" — the invasion of the southern Philippine Islands.

Myōkō was hit by one 500-pound (227 kg) bomb, causing only superficial damage, but she was drydocked at Sasebo Naval Arsenal for repairs.

In the Battle of the Java Sea on 1 March 1942 Myōkō, Nachi, and Haguro participated in the destruction of the last remaining Allied fleet units in the Netherlands East Indies.

[7] In June, Myōkō was part of Vice Admiral Nobutake Kondō's support force in the Battle of Midway, which included the battleships Kongō and Hiei, the heavy cruisers Haguro, Atago and Chōkai, the light cruiser Yura, the light aircraft carrier Zuihō, and seven destroyers.

From Rabaul, Myōkō sailed with the light cruisers Agano and Sendai and six destroyers to escort reinforcements to the island of Bougainville.

The warships sailed ahead of the transports and engaged an American force in the Battle of Empress Augusta Bay at 12:50 on 3 November.

Haguro had received minor damage in the action, and the American destroyer USS Foote was crippled by a Long Lance torpedo.

On 10 February, while sailing from Truk to Palau with Atago and Chōkai and eight destroyers, Myōkō was attacked by the submarine USS Permit.

This battle was later called the "Great Marianas Turkey Shoot" by American sailors, because over 300 Japanese carrier aircraft were shot down in a single day on 19 June.

En route to Cam Ranh Bay, Myōkō was hit by one torpedo from a spread of six, fired by the submarine USS Bergall at 17:35 on 13 December 1944 on her aft port side, blowing away her stern, and leaving her unable to steer.

Unable to steer, she was towed by destroyer Ushio (which assisted in damaging Bergall, which survived and returned to Fremantle) and several other ships to Singapore harbor for repairs, but materials in Singapore were insufficient to complete the repairs for both Myōkō and Takao, the latter which had been severely damaged by two submarine-launched torpedoes prior to the Sibuyan Sea battle.