Maidstone led the 8th Submarine Flotilla to its war station at Harwich in the 1914 mobilisation. She remained the principal depot ship for the offensive submarine force at Harwich for the duration of the hostilities. Maidstone's Flotilla was re-established as the 9th Submarine Flotilla in August 1916. However, this was only a change of the flotilla number and there was no change in her duties.