French battleship Mirabeau

The four aft 75-millimeter gun positions were very prone to flooding in anything other than calm weather and were removed from the Dantons in 1915–1917; their embrasures were plated over to stop the flooding. The four 47-millimeter guns on the aft superstructure were transferred to other ships at some point during the war. Those on the forward superstructure received high-angle mounts and were repositioned to better serve as anti-aircraft guns.[4]

Before the French declared war on the German Empire on the morning of 4 August, Vice Admiral (Vice-amiral) Augustin Boué de Lapeyrère, commander of the 1st Naval Army (1^re Armée Navale), as the Mediterranean Squadron had been renamed in 1913, ordered most of his ships to concentrate off the Algerian coast to escort troop convoys to Metropolitan France and to blockade German shipping in the Mediterranean. Despite this deployment the German battlecruiser SMS Goeben and light cruiser SMS Breslau were able to bombard Bône and Philippeville on 4 August. Despite Mirabeau's machinery problems which limited the 1st Battle Squadron to a speed of 15 knots (28 km/h; 17 mph), the squadron probably would have intercepted the German ships later that morning if its commander, Vice Admiral Paul Chocheprat, had not ordered a turn to the west.[15] Mirabeau spent the rest of the month under repair.[11] Aside from several uneventful sorties into the Adriatic, the French capital ships spent most of their time cruising between the neutral Greek and Italian coasts[16] to prevent the Austro-Hungarian fleet from attempting to break out of the Adriatic.[17] The torpedoing of the dreadnought battleship Jean Bart on 21 December by the Austro-Hungarian submarine U-12 showed that the battleships were vulnerable to this threat, and they were withdrawn to spend the rest of the month further south at an anchorage in Navarino Bay.[18]

On 11 January 1915, the French were alerted that the Austro-Hungarian fleet was going to sortie from its base at Pola, so the Naval Army sailed north to the Albanian coast. It proved to be a false alarm, and they were back at their moorings three days later. In the meantime, the ships patrolled the Ionian Sea. The declaration of war on Austria-Hungary by Italy on 23 May, and the Italian decision to assume responsibility for naval operations in the Adriatic, allowed the French Navy to withdraw to either Malta or Bizerte, Tunisia, to cover the Otranto Barrage. Mirabeau was in Malta on 30 June when a torpedo exploded in a nearby warehouse. The ship was only lightly damaged, but two men of her crew were killed and eleven wounded.[19]

Mirabeau bombarding Athens, 1 December 1916

In November 1916, she transported landing parties from the other ships of her division to Athens. On 1 December, those landing parties participated in the Allied attempt to ensure Greek acquiescence to Allied operations in Macedonia. Greek resistance to the Allied action ended after Mirabeau fired four rounds from her main armament into the city, one of which landed near the Royal Palace. Afterwards, she spent 1917 based at Corfu or at Mudros to prevent Goeben, by this time transferred to the Ottoman Empire and renamed Yavuz Sultan Selim, from breaking out into the Mediterranean.[20] In August the ship tested a tethered balloon, but it was destroyed by a lightning strike on 31 October.[21] In April 1918, Mirabeau accompanied her sisters Diderot and Vergniaud to Mudros, where they remained for the rest of the war.[20] The 1st Naval Army was reorganized on 1 July and most of the Danton-class ships were assigned to the Second Squadron (2^e Escadre) with Mirabeau part of the First Division of the squadron.[22]

One of the ship's main guns being reinstalled in Sevastopol in 1919

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 beginning on 12 November. By 8 December Mirabeau was already deployed to the Black Sea to support White Russian forces in Sevastopol and deter Soviet forces who were advancing on the city during the Russian Civil War.[23] Mirabeau ran aground during a snowstorm on 18 February 1919 off the Crimean coast.[20] She could not be refloated until 6,000 metric tons (5,900 long tons) of weight were removed. To this end, her guns, turret armour, most of her boilers, coal, ammunition and the upper strake of belt armor was disembarked. After several weeks of effort, the ship was refloated on 6 April and she was towed by the Russian cruiser Kagul and the tugboat Thernomore to a drydock in Sevastopol to repair any damage and to have her equipment reinstalled. This process began on 12 April, but Mirabeau was refloated three days later as news was received of the Red Army's advance on the city. Only her guns, ammunition and part of her armor had been replaced by the time the city was evacuated by the Allies on 5 May. She was towed to Constantinople by the battleship Justice and five tugs where she remained from 7–15 May. Justice then resumed the tow and they reached Toulon on the 24th. After an inspection of the ship in a drydock, Mirabeau was stricken on 22 August and turned over as compensation to the company that salvaged the pre-dreadnought battleship Liberté. Her hulk was towed to Savona, Italy, to be scrapped on 28 April 1922.[24]