Cornwallis supported the Landing at Cape Helles on 25 April and shelled Ottoman troops over the following month as the Allied soldiers sought to push further inland.

The Duncans proved to be disappointments in service, owing to their reduced defensive characteristics, though they were still markedly superior to the Peresvets they had been built to counter.

The Duncan-class ships were powered by a pair of 4-cylinder triple-expansion engines that drove two screws, with steam provided by twenty-four Belleville boilers.

[7] When the First World War began in August 1914, plans originally called for Cornwallis and battleships Agamemnon, Albemarle, Duncan, Exmouth, Russell, and Vengeance to combine in the 6th Battle Squadron and serve in the Channel Fleet, where the squadron was to patrol the English Channel and cover the movement of the British Expeditionary Force to France.

[7][8] Cornwallis and her sisters, as well as the battleships of the King Edward VII class, were temporarily transferred to the Channel Fleet on 2 November to reinforce the latter in the face of Imperial German Navy activity in its area.

She departed Portland on 24 January 1915 and arrived at Tenedos to join the British Dardanelles Squadron under Admiral Sackville Carden on 13 February 1915.

[7] Cornwallis was one of six British and French battleships selected to lead the first attack on the straits on 19 February, under the operational control of Vice Admiral John de Robeck.

At 09:51 on 19 February, Cornwallis fired the first shots of the Dardanelles campaign when she began her bombardment of the "Orkanie" battery, but her time on station was cut short when a defective capstan prevented her from dropping anchor.

About an hour and a half later, the Ottoman coastal guns finally began to engage the Anglo-French fleet, and Cornwallis came under fire but was unscathed.

[9] A second attack began on 25 February; de Roebeck, aboard Vengeance, was to lead the assault in company with Cornwallis, followed by the French Admiral Émile Paul Amable Guépratte with Suffren and Charlemagne.

The other ships began shelling the Ottoman fortresses in the late morning, and de Roebeck was given the order to begin his run into the narrows at 12:15.

Cornwallis followed Vengeance at a distance of four cable lengths and the two ships made their initial pass into the straits before turning about to allow Guépratte room to manoeuvre.

Neither ship was damaged in the attack and de Roebeck reported that several of the Ottoman batteries were no longer manned, so Guépratte began his run.

His ships received only a single shot in return, so Carden ordered a group of minesweepers to enter the straits and begin clearing the naval mines.

The landing parties succeeded in destroying several guns at Kumkale, "Orkanie", and Sedd el Bahr, but the work was not completed by the end of the operation that day.

[12] The British attempted another raid on 4 March; Cornwallis was stationed inside the strait to directly support a landing party of Royal Marines from the troopship SS Braemar Castle.

Poor visibility and harassing fire from mobile Ottoman field guns prevented Queen Elizabeth from inflicting serious damage, and the attack was called off.

[14] Early on 10 March, Cornwallis, Irresistible, and the seaplane carrier Ark Royal went to join Dublin in the Gulf of Saros, where they were to reconnoitre the Ottoman defences further up the Gallipoli peninsula.

The weather proved to be too bad for Ark Royal's seaplanes to operate, so Cornwallis shelled the town of Bulair before leaving for Tenedos.

By 10:00, the British troops had secured a beachhead, so Cornwallis left to support River Clyde that had been beached under heavy fire at Sedd el Bahr, but by this time, the decision had been made to refrain from landing the men stranded aboard River Clyde until nightfall, owing to the stiff Ottoman resistance.

[17] As the Allied ground forces advanced on Krithia on 28 April, Cornwallis and several other battleships were assembled to provide fire support for the attack.

[7][20] After the Suvla Bay evacuation was complete, Cornwallis was transferred to the Suez Canal Patrol in company with the battleship Glory and Euryalus, which they joined on 4 January 1916.

On 9 January 1917, Cornwallis was hit on her starboard side by a torpedo from German U-boat U-32, commanded by Kurt Hartwig, in the eastern Mediterranean, 60 nautical miles (110 km; 69 mi) east of Malta.