When World War I began in August 1914, she was assigned to the Cape Verde Station to search for German commerce raiders while protecting British shipping.

She was assigned to the North America and West Indies Station in 1915 and continued to patrol against German raiders and escort convoys to the end of the war.

She was powered by two 4-cylinder triple-expansion steam engines, each driving one shaft, which produced a total of 21,000 indicated horsepower (16,000 kW) and gave a maximum speed of 22 knots (41 km/h; 25 mph).

[2] Her main armament consisted of four breech-loading (BL) 7.5-inch Mk I guns mounted in four single-gun turrets, one each fore and aft of the superstructure and one on each side.

On 31 July, a few days before war was declared on Germany, she encountered the German light cruiser Strassburg in the English Channel returning home and the two ships saluted each other.

[10] When news of the outbreak of hostilities was received on 5 August, Carnarvon, now the flagship of Rear Admiral Archibald Stoddart, was at sea, making for the Canary Islands from Gibraltar.

Vice-Admiral Maximilian von Spee, commander of the German squadron, had other plans and intended to destroy the radio station at Port Stanley on the morning of 8 December.

Carnarvon, now 10 nautical miles (19 km; 12 mi) behind, had no hope of catching the scattering German ships and continued to trail the battlecruisers.

[13] Carnarvon finally came within range of the German armoured cruisers and opened fire shortly before Scharnhorst rolled over and capsized at 16:17.

[10] Carnarvon received permanent repairs in Montreal, Canada, from May to July after which she escorted several British H-class submarines from Halifax to the United Kingdom en route to Devonport.

[10] Now assigned to the North America and West Indies Station, with its main base at the Royal Naval Dockyard in the Imperial fortress colony of Bermuda, she resumed her duties protecting British shipping for the rest of the war.