Mersey had a relatively successful career in the First World War and had two prominent incidents.

At the Battle of the Yser in 1914, off the coast of Belgium, she bombarded German troops as well as artillery positions.

[note 1] On 6 July 1915 the two monitors undertook an extended engagement with the Königsberg, which had taken shelter in the shallow waters of the Rufiji Delta.

Returning on 11 July the two monitors caused extensive damage to the German ship, forcing it to be scuttled.

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