The ship participated in most of the early fleet actions, including the Battles of Heligoland Bight, Dogger Bank, and Jutland, but was only seriously engaged in the latter.

[10] On the outbreak of the First World War in August 1914, she was assigned to the 1st Light Cruiser Squadron of the Grand Fleet.

On 28 August 1914 she was present, but played no significant role, in the Battle of Heligoland Bight,[11] Several months later, the Germans bombarded Scarborough, Hartlepool and Whitby on 16 December and the 1st LCS was escorting Vice-Admiral David Beatty's battlecruisers in response when it encountered a German light cruiser and a half-flotilla of torpedo boats.

[12] On 28 December, the ship was transferred to the 3rd LCS and became the flagship of Rear-Admiral Trevylyan Napier, the squadron commander.

As Beatty turned east to rendezvous with them, the cruiser engaged several German ships at ranges below 7,000 yards (6,400 m) and in poor visibility.

[14] By 20:10, Beatty's ships were in front of the Grand Fleet and the 3rd LCS was screening them when Falmouth spotted five cruisers of the 4th Scouting Group and the squadron closed to engage at full speed.

Despite poor visibility, she hit the light cruiser SMS München twice, one of which damaged her aft boilers and impaired her ability to keep steam up.

The German objective was to bombard Sunderland the following day, based on extensive reconnaissance conducted by Zeppelins and submarines.

Part of the German plan was to draw the British ships through a series of submarine ambushes and Falmouth fell victim to one of the awaiting U-boats, U-66, about 16:05 the following afternoon after the Grand Fleet was headed for home.

Two torpedoes only badly damaged the cruiser, but follow-on attacks were unsuccessful due to the presence of the escorting destroyers.

Falmouth was able to steam through the night under her own power at a speed of 2 knots (3.7 km/h; 2.3 mph), although a pair of tugboats came out the following morning and took her in tow.