Shortly after World War I began, the ship participated in the Battle of Antivari in the Adriatic Sea and helped to sink an Austro-Hungarian protected cruiser.

This, combined with other poor traits, including the great weight in coal they had to carry, making them rather unsuccessful ships, though their numerous rapid-firing guns were of some use in the Mediterranean.

[2] She carried a maximum of 2,027 tonnes (1,995 long tons) of coal which allowed her to steam for 3,370 nautical miles (6,240 km; 3,880 mi) at a speed of 10 knots (19 km/h; 12 mph).

The ship participated in combined fleet maneuvers between Provence and Tunisia in May–June 1913[5] and the subsequent naval review conducted by the President of France, Raymond Poincaré on 7 June 1913.

[5] In early August 1914, the ship cruised the Strait of Sicily in an attempt to prevent the German battlecruiser Goeben and the light cruiser Breslau from breaking out to the West.

In May 1918, Diderot became flagship of the First Division of the Second Squadron and was transferred that month to Mudros with her sister ships, Mirabeau and Vergniaud, to prevent Goeben from breaking out into the Mediterranean and remained there until the end of the war.

[5] After the Armistice of Mudros was signed on 30 October between the Allies and the Ottoman Empire, the ship participated in the early stage of the occupation of Constantinople from 12 November to 12 December.