At the start of the Second World War in 1939, Cumberland was assigned to 2nd Cruiser Squadron Force G, the South American Division.

Without her, HMS Exeter, Ajax and Achilles engaged the German raider Admiral Graf Spee in the Battle of the River Plate on 13 December.

Cumberland received a garbled indication that a contact was being made and moved north to reinforce, arriving at the River Plate at 22:00 on 14 December, after steaming 1,014 nmi (1,878 km; 1,167 mi) in 34 hours, at 29.8 kn (55.2 km/h; 34.3 mph).

Whilst on patrol, she intercepted the Vichy French merchant Poitiers, which had been carrying ammunition to the Ivory Coast.

Although she was without her 8-inch gun turrets at this time and was refitted with lattice masts, she is very recognisable as the last of the three-funnelled heavy cruisers to remain in service.

Her first voyage in this configuration was delayed after some "defects" were found in her engine room, which were not explained as normal mechanical faults.

In April 1956, having set sail on another secret test mission, she returned to port within 36 hours, following another unexplained "defect" in her main gearbox.