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 (1ʳᵉ 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.
Mirabeau spent the rest of the month under repair.
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 to prevent the Austro-Hungarian fleet from attempting to break out of the Adriatic.
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by the Austro-Hungarian submarine U-12 showed that the battleships were vulnerable to this threat, and they were withdrawn to spend the rest of the month further south at an anchorage in Navarino Bay.

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.

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.
In August the ship tested a tethered balloon, but it was destroyed by a lightning strike on 31 October.
In April 1918, Mirabeau accompanied her sisters Diderot and Vergniaud to Mudros where they remained for the rest of the war.
The 1st Naval Army was reorganized on 1 July and most of the Danton-class ships were assigned to the Second Squadron (2ᵉ Escadre) with Mirabeau part of the First Division of the Squadron.
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 (1ʳᵉ 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. Mirabeau spent the rest of the month under repair. 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 to prevent the Austro-Hungarian fleet from attempting to break out of the Adriatic. 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.
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. 
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. In August the ship tested a tethered balloon, but it was destroyed by a lightning strike on 31 October. In April 1918, Mirabeau accompanied her sisters Diderot and Vergniaud to Mudros where they remained for the rest of the war. The 1st Naval Army was reorganized on 1 July and most of the Danton-class ships were assigned to the Second Squadron (2ᵉ Escadre) with Mirabeau part of the First Division of the Squadron.
French battleship Mirabeau