Commissioned shortly before the start of World War II in August 1939, the ship was initially assigned to the China Station and was transferred to the Indian Ocean and later to South Africa to search for German commerce raiders.

[8] During sea trials off the coast of Talland, Cornwall, in November 1938, the cruiser recorded an average speed at a standard displacement of 31.92 knots (59.12 km/h; 36.73 mph) against the measured mile.

However, trials at Talland generally resulted in lower speeds than at other locations due to its shallow depth affecting propulsion efficiency.

[10] Following completion on 31 January 1939, the ship was assigned as the flagship of the 4th Cruiser Squadron (CS) on the China Station, where she served until the beginning of World War II in September.

[11] In mid-November, Gloucester and the French aviso Rigault de Genouilly were assigned to patrol the Indian Ocean between Madagascar and Seychelles in an unsuccessful search for the German commerce raider Admiral Graf Spee.

[13] A few days after Italy joined the war on 10 June, Gloucester and her sister ship, Liverpool, bombarded Tobruk, Libya, sinking a small auxiliary minesweeper on the 12th.

The next day, a bomb dropped by Savoia-Marchetti SM.79 bombers struck the ship's bridge, killing 18 crew members instantly, including the captain.

Despite an inoperable bridge, the ship remained with the fleet and participated in the Battle of Calabria on the 9th, although she was ordered away from the battleline to escort the aircraft carrier Eagle.

[18] On 11 January 1941, while supporting Operation Excess (several coordinated convoys), Gloucester and sister ship Southampton came under attack from Junkers Ju 87 "Stuka" dive bombers from StG 2 while leaving Malta.

Southampton was hit by at least two bombs and caught fire; heavily damaged and without power, the ship was scuttled by torpedoes from the light cruiser Orion.

[19] On 27 March, Gloucester, now reassigned to the 7th CS, departed Piraeus bound for Souda Bay, Crete as part of Vice-Admiral Andrew Cunningham's plan to trap and destroy a large portion of the Italian Fleet which was at sea in an attempt to intercept British convoys operating between Greece and Egypt.

British signals intelligence had revealed the Italian plan and Cunningham attempted to consolidate his ships, but was delayed and missed his rendezvous with the 7th CS scheduled for the following morning.

Although Vice-Admiral Henry Pridham-Wippell knew that the Italians had a battleship at sea, he was still caught by surprise when Vittorio Veneto opened fire at 10:55 at Orion.

The 3rd Cruiser Division joined in shortly afterwards, but gunnery problems plagued the Italians and they scored no hits against their primary targets, Gloucester and Orion.

After covering another convoy to Malta, the ship, together with the battleships Warspite, Valiant, and Barham, and various destroyers, attacked Tripoli harbour on the night of 20/21 April with some success.

At the end of the month, the ship was briefly transferred to Force H at Gibraltar before escorting a convoy eastward to Malta and rejoining the Mediterranean Fleet in Operation Tiger in early May.

Writing in despatches after the battle, Cunningham stated that King was unaware of the shortage of anti-aircraft ammunition in Fiji and Gloucester.

[23][25] While in the Kythira Strait, about 14 miles (12 nmi; 23 km) north of Crete, Gloucester and Fiji were attacked by "Stuka"s of StG 2.

"[1] On 30 May 1941, in a letter to the First Sea Lord, Sir Dudley Pound, Cunningham wrote, "The sending back of Gloucester and Fiji to the Greyhound was another grave error and cost us those two ships.