[9][10] On 23 January 1915, the German battlecruisers under Admiral Franz von Hipper made a sortie to attack British fishing boats on the Dogger Bank.

[20][21] On 23 March 1915 ships of the Harwich Force escorted the seaplane carrier Empress on an attempted raid against a German radio station at Norddeich.

The force ran into thick fog just as the seaplanes were due to be launched, causing the operation to be abandoned, and the destroyer Landrail collided with the light cruiser Undaunted.

Landrail was badly damaged, with her bow smashed, and Mentor helped to tow the stricken destroyer stern-first back to Harwich.

[22][23] On 28 March 1915, four destroyers of the Harwich force (Laurel, Liberty, Leonidas and Lucifer) carried out an anti-submarine sweep off the Dutch coast.

[25][26] On 2 July 1915, Mentor, together with Manly and Miranda escorted the former ocean liner Empress of Britain, on passage from Liverpool to the Dardanelles on the first part of her journey and carrying 4500 troops.

Mentor and Miranda left Empress of Britain at about 5:00pm that evening so they could turn back and pick up Aquitania, another Dardanelles-bound former ocean liner, carrying 5939 tropps which set out from Liverpool on 3 July.

[27] Later that morning, Mentor and Miranda picked up an SOS call from the transport Anglo-Californian, carrying a load of 927 horses from Canada to Britain.

[28][29][30] On 4–5 August 1915, Mentor took part in a sweep of four light cruisers and four destroyers of the Harwich Force against German torpedo-boats and trawlers that were believed to be patrolling off Terschelling.

On the afternoon of16 August, the force encountered groups of neutral and German trawlers., and detected suspicious radio signals which appeared to come from the vicinity.

The operation continued, despite the knowledge from radio intercepts that a force of German destroyers (II Torpedo-Boat Flotilla) was at sea.

Princess Margaret turned away to avoid the attack, with the rest of the British destroyers (most of which had not spotted the German ships and thought that Mentor had struck a mine) following.

[32][33] On 25 December 1915, Mentor was one of eight destroyers from the Harwich Force that were ordered with the leader Nimrod to the Channel as a result of attacks by the German submarine U-24.

[44][45] On 5 June 1917, Mentor was part of the escort for the monitors Erebus and Terror when the bombarded the German-held port of Ostend.

[48] On 22 April 1918, the British launched attacks against Zeebrugge and Ostend, with the intention of blocking the entrances to the canals linking these ports with Bruges.