At the outbreak of the Second World War, Achilles began patrolling the west coast of South America looking for German merchant ships, but by 22 October 1939 she had arrived at the Falkland Islands, where she was assigned to the South American Division under Commodore Henry Harwood and allocated to Force G (with Exeter and Cumberland).

The range reduced to about 4 nautical miles (7.4 km) at around 07:15 and Graf Spee broke off the engagement around 07:45 to head for the neutral harbour of Montevideo which she entered at 22:00 that night, having been pursued by Achilles and Ajax all day.

Faced with what he believed to be overwhelming odds, the captain of Graf Spee, Hans Langsdorff, scuttled his ship rather than risk the lives of his crew.

[4] After Japan entered the war, she escorted troop convoys, then joined the ANZAC Squadron in the south-west Pacific.

Her single 4-inch AA guns were replaced by the dual-purpose QF 4 inch Mk XVI naval gun in four twin mountings, modern radar was fitted, and the damaged X turret was replaced by four QF 2 pom poms in a quadruple-mount.

Stoker William Dale was awarded the Albert Medal for Lifesaving for his actions in saving the lives of several dockyard workers.

In 1968 she was present at the granting of independence to Mauritius representing the Indian Government together with the Royal Navy frigate Tartar under Captain Cameron Rusby.

[8] On 22 January 1979, Admiral Jal Cursetji, the Indian Navy Chief of the Naval Staff, presented Achilles's builder's plaque, steering wheel and engine room telegraph to Admiral Terence Lewin, the First Sea Lord and Chief of the Naval Staff of the Royal Navy.