The Mogami-class ships were constructed as "light cruisers" (per the London Naval Treaty) with five triple 155-millimetre (6.1 in) dual purpose guns.

They were exceptionally large for light cruisers, and the barbettes for the main battery were designed for quick refitting with twin 8-inch (203 mm) guns.

[3] Mogami served in numerous combat engagements in World War II, until she was sunk at the Battle of Leyte Gulf in October 1944.

This resulted in the choice of the dual purpose (DP) 15.5 cm/60 3rd Year Type naval guns as the main battery in five triple turrets capable of 55° elevation.

[4] The class was designed from the start to be upgraded into heavy cruisers with the replacement of their main battery with 20 cm/50 3rd Year Type naval guns in twin turrets.

At the time of the attack on Pearl Harbor, Mogami was assigned to cover the Japanese invasion of Malaya as part of Cruiser Division 7 under Vice Admiral Jisaburo Ozawa's First Southern Expeditionary Fleet, providing close support for landings of Japanese troops at Singora, Pattani and Kota Bharu.

Both Houston and Perth were sunk during the engagement, as was Japanese transport Ryūjō Maru with IJA 16th Army commander Lieutenant General Hitoshi Imamura aboard.

On 26 May, Cruiser Division 7 arrived at Guam to provide close support for Rear Admiral Raizō Tanaka's Midway Invasion Transport Group.

[5] On 5 June, Admiral Isoroku Yamamoto, CINC of the Combined Fleet ordered Cruiser Division 7 to shell Midway Island in preparation for a Japanese landing.

At 0534, retiring Mikuma and Mogami were bombed from high altitude by eight USAAF Boeing B-17 Flying Fortresses from Midway, but they scored no hits.

At 0805, six USMC Douglas SBD Dauntless dive-bombers and six Vought SB2U Vindicators from Midway attacked Mikuma and Mogami but they only achieved several near-misses.

[5] The following morning, 6 June 1942, Mikuma and Mogami were heading for Wake Island when they were attacked by three waves of 31 SBD Dauntless dive-bombers from the aircraft carriers USS Enterprise and Hornet.

Mogami returned to Japan, and underwent a major conversion at Sasebo Naval Arsenal from 25 August to an aircraft cruiser to improve the fleet's reconnaissance capabilities.

Her aft deck was extended and fitted with a rail system to accommodate the planned stowage of 11 Aichi E16A Zuiun ("Paul") reconnaissance floatplanes.

While at anchor at Rabaul on 5 November, Mogami was attacked by a SBD Dauntless dive-bomber from the aircraft carrier USS Saratoga and hit by a 500 lb (227 kg) bomb.

[5] On 13 June 1944, Admiral Soemu Toyoda, CINC, Combined Fleet, activated the "A-Go" plan for the defense of the Mariana Islands.

Later in the day, the Mobile Fleet's aircraft attacked Task Force 58 off Saipan, but suffered overwhelming losses in the "Great Marianas Turkey Shoot".

At 2030 on 20 June, two hours after Hiyō was hit by torpedoes by Grumman TBM Avengers from the aircraft carrier USS Belleau Wood, she exploded and sank.

On 8 July, Mogami departed Kure via Okinawa and Manila back for Singapore and Brunei, and was involved in fleet training and patrols in the Singapore-Brunei area through October.

In the morning of 24 October 1944, Vice Admiral Nishimura ordered the launch of Mogami's floatplane to reconnoiter Leyte Gulf.

The plane reported sighting four battleships, two cruisers and about 80 transports off the landing area and four destroyers and several torpedo boats near Surigao Strait.

Between 0350–0402 hours on 25 October, after entering the Surigao Strait, Mogami was struck by four 8-inch (203 mm) shells from the heavy cruiser USS Portland, which destroyed both the bridge and the air defense center.