The County-class was the first post-war cruiser class built for the Royal Navy, and also the first to be designed within the limits of the Washington Treaty which set a maximum displacement for cruisers at 10,000 tons. Unlike contemporary US and Japanese cruisers which prioritized firepower, the County-class was designed for range and seaworthiness, and therefore featured a strong, long hull with a flush deck and high freeboard. Armament consisted of eight BL 8-in Mk. VIII guns in four twin turrets. There proved initially problematic due to them being designed as dual-purpose guns; in practice they were rarely used in the anti-aircraft role. On the positive side, their high rate of fire compensated for the fewer number of guns. Anti-aircraft protection consisted of 4-in and 2-pounder guns, and was considerably upgraded in World War II. The main drawback of the County class ships was that armor protection was reduced to a minimum in order to stick to the Treaty limits, . For example, belt armor was limited to just one inch which only protected against shell fragments, although a maximum of 4.5 inches was fitted to vital machinery. A total of three sub-classes were built, with minor design changes. The initial London group were characterized by external torpedo bulges and short funnels (raised shortly after completion); the Kent group (seven ships) had internal bulges and different armor arrangement; the final Norfolk group (four ships) had a lowered bridge. A further Surrey group (two ships) was cancelled as construction priorities were shifted towards a larger number of light cruisers. Most ships received an extensive reconstruction in the 1930s, where they were enabled to carry aircraft, given extra armor and anti-aircraft armament, and eventually were also provided with radar (among the first Royal Navy ships to be equipped as such). All thirteen ships (including two that were built for the Royal Australian Navy) saw extensive action in World War II although three of them were sunk in the Pacific either by aircraft or gunfire. The surviving ships were retired post-war.

HMS Kent operated in the Mediterranean early in the war and later participated in operations against the Tirpitz, along with the HMS Berwick. HMS Devonshire participated in the Norway campaign and evacuated the Norwegian royal family and government to the UK. It later sunk the German commerce raider Atlantis in the South Atlantic. HMS Norfolk and HMS Suffolk participated in the Battle of the Denmark Strait and subsequently trailed the Bismark over the following days. They would eventually be joined by HMS London and HMS Dorsetshire. The HMS London also participated in the Chinese civil war, where it was damaged in the Amethyst Incident. Two of the ships, HMS Cornwall and HMS Dorsetshire, were also present during the Japanese attacks on Ceylon and sunk by aircraft. Other Royal Navy ships that participated in the Pacific and Indian Oceans included HMS Cumberland, HMS Kent, andHMS Sussex where the surrender of Singapore garrison was signed in September 1945. The RAN ships included HMAS Australia (which participated in most major Pacific theater naval battles) and HMAS Canberra, the latter which was sunk during the Battle of Savo Island. Consequently, the HMS Shropshire was transferred to the RAN in 1942.