Joseph-Simon was born on the 24th April 1849 in Saint-Béat. He was the son of Lieutenant Gallieni, an Italian who had enlisted in a foreign regiment in 1829 and become a neutralised French citizen in 1841, before commanding the garrison at Val d'Aran. A student at the Prytanee Military Academy in La Flèche, he then attended the Special Military School of St Cyr in 1868. On the 15th July 1870, four days before the beginning of the Franco-Prussian war, he was assigned as a sub-lieutenant to the 3rd regiment of marine infantry. On the 30th August he received the order to defend the Hauts-de-Meuse between Mouzon and Carignan. Taken prisoner in September and imprisoned for seven months alongside the members of his unit who had survived the Battle of Bazeilles, he was held in Ingolstadt prison and then in Magdebourg and released on the 12th March 1871. On returning from captivity, he was posted to Paris and then to Rochefort, before embarking for the island of Reunion on the 1st April 1872, where he served for three years as a lieutenant. In June 1875, he returned to metropolitan France and joined the 1st Regiment of Marine Infantry in December 1876. Posted to Goree Island, he later became commander of Thiès where he distinguished himself through his diplomatic and strategic qualities and his desire to "reach the souls of the people". At headquarters in Senegal, General Brière de l'Isle assigned him to exploration missions to the valleys of the Senegal and Niger rivers and in the Sudan. On the 29th March 1880, through the treaty of Bafoulabe, he established the French protectorate on Mali. The following year he negotiated the French protectorate treaty on the Niger with Sultan Ahmadou.
In June 1881, on leaving the 3rd Infantry Regiment, he returned to Paris before taking up his duties as Chief of Battalion in Toulon in March 1882. After three years spent in Martinique (1883-1886), in May 1886 Lieutenant Colonel Gallieni set off for the Sudan, where he was appointed Superior Commander on the 20th December, in order to re-establish order in the upper valley of the Senegal. In July 1888 he was Commander of the 4th Regiment of Marine Infantry in Toulon, followed by the 8th Regiment in April 1890. A graduate of the École de guerre (war academy), in March 1891 he was promoted to Colonel at the headquarters of the Marine Infantry. From September 1892 to August 1896, he took part in the pacifying campaigns in Tonkin at the head of the 3rd regiment of Tonkinese tirailleurs. Supported by Commander Lyautey, he pursued pirates on the China Sea and laid the foundations of the French colonial system. As Brigade General in 1896, at the request of the Minister for the colonies André Lebon, Gallieni was sent to Madagascar, becoming Governor General there in September. He forced Queen Ravanalo the Third to abdicate and established the island's economic development programme. Promoted to Division General in 1899, his duties brought him back to Paris before his return to Madagascar in June 1900. Gallieni was appointed Commander in Chief of the troops of the Eastern Africa group in 1903, two years before returning to France and resigning from his post of Governor General.
In February 1906 he was commander of the 13th army corps in Clermont-Ferrand and then promoted to Military Governor of Lyon in June and appointed at the head of the 14th Army Corps. Assigned to the Upper War Council and the Presidency of the provisional council for the defence of the colonies in 1908, he carried out inspection visits in North Africa. Elected to the Upper Council for National Defence in 1911, he gave Joffre the opportunity to become Commander in Chief of the French Army. Reaching the upper age limit in April 1914, the General was recalled on the 26th August by Viviani's government to take command of the armies' entrenched Paris camp. He totally reorganised the defence of the capital and secured reinforcements from Maunoury's 6th Army, the 45th Algerian Division and the 4th Corps of the 3rd Army. On the 2nd September, he received full civilian and military powers. He provided a new impetus for national defence, most notably by requisitioning all automobiles and horse drawn vehicles, as well as the Parisian taxis which would play a decisive role in the counter-offensive that liberated Paris from the threat of the army of the Reich. On the 8th September, Maunoury returned under Joffre's orders. Gallieni however retained his authority over Paris throughout the month of November and dissolved his civilian cabinet on the 7th December. On his return to government, Aristide Briand offered him the portfolio of War Minister on the 29th October 1915. It was a difficult task: to improve the effectiveness of the High Command and reorganise the operation of its administration. On the 2nd December the post of Commander in Chief of all the armies was created and given to him. Challenged by the President of the Council following his suggestion to reform the High Command on the 7th March 1916, he resigned three days later for health reasons. He was admitted to hospital in Versailles in April 1916 and died on the night of the 27th to 28th May. Following a state funeral, the "Saviour of Paris" was buried at the Saint-Raphaël cemetery. Joseph Gallieni was posthumously made Marshal France on the 6th May 1921. He held the Great Cross of the Légion d'honneur and the Military Medal.