During World War I intelligence reports suggested that the Germans were building a new class of cruiser which could outgun the existing C-class light cruisers. It was believed that an improved C class with an added super-firing gun in front of the deckhouse would maintain British naval superiority in a battle. In September 1916 the first three ships of the new class were launched and a second group was ordered in July 1917. Diomede, was part of a third tranche ordered in March 1918. This group was to have comprised six vessels, but the end of the war saw this reduced to two, Diomede and. Diomede had been laid down Vickers Armstrong of Barrow-in-Furness in mid-1918, but after launching in 1919 work was suspended. However, in 1922 she was towed to the Royal Dockyard, Portsmouth and completed on 22 February. Like the second group of Danae-class vessels, Diomede was fitted with a trawler bow. Diomede was unique in her class in having an experimental fully enclosed 6-inch mounting in the "A" position. Before completion, the suitability of her conversion to a royal yacht had been discussed, but nothing came of the idea.
Early career
Upon commissioning Diomede joined the 5th Light Cruiser Squadron on the China Station in 1922. In 1925 she was transferred to the New Zealand Division of the Royal Navy at Devonport where she served until 1935, apart from a refit in 1929-1930. In 1931 she rendered assistance to the town of Napier, New Zealand after the devastating Hawkes Bay earthquake, supplying medical personnel, equipment, guards and firemen, along with her sister ship Dunedin. Afterwards Diomede escorted the beach-damaged sloop to Auckland. The Executive officer at the time was Commander, later Admiral Victor Crutchley, who was to later become entwined with the Pacific Campaign of World War II. Upon the notification that the two cruisers of the New Zealand Division were to be replaced by cruisers, in 1935 Diomede started her voyage home to Britain to be paid off into the reserve. En route the Abyssinian Crisis broke out and she was diverted to the 4th Light Cruiser Squadron, part of the East Indies Station based at Aden for possible action against the Italians. Upon relief by the cruiser on 31 March 1936 she was paid off and spent the next three years in the reserve fleet or as a troop ship.
World War II
With the growing likelihood of war, Diomede was reactivated. 3 September 1939 found her with the 7th Cruiser Squadron serving as part of the Home Fleet on Northern Patrol duties. Before the despatch of the 7th Cruiser Squadron to the Mediterranean in 1940 in light of the Italian threat, Diomede was attached to the 8th Cruiser Squadron on the America and West Indies Station on shipping protection and patrolling duty. On 8 December 1940 she chased the German blockade runner Idarwald from Tampico, Mexico. Before a capture could be effected the crew of the German freighter set it afire and scuttled it off Cabo Corrientes, Cuba. The US destroyer observed the proceedings. In early 1942 Diomede joined the 9th Cruiser Squadron as part of the South Atlantic and West African Squadron. After more than a decade of steaming it was decided to retire her from front-line service and from 22 July 1942 to 24 September 1943 at Rosyth Dockyard she converted into a training ship. With the end of the war she was reduced to reserve. On 5 April 1946 she was sold for scrap to Arnott Young of Dalmuir and arrived there for breaking up on 5 May.
