Soult was born at Saint-Arnans-la-Bastide (now Saint-Amans-Soult near Castres in the Tarn département), the son of a country notary Jean Soult (1726 – 1779) and wife Brigitte de Grenier, paternal grandson of Jean Soult (1698 – 1772) and wife Jeanne de Calvet and maternal grandson of Pierre François de Grenier de Lapierre and wife Marie de Robert. He was well-educated, and intended to become a lawyer, but his father's death when he was still a boy made it necessary for him to seek his fortune, and he enlisted as a private in the French infantry in 1785.

His superior education ensured his promotion to the rank of sergeant after six years' service, and in July 1791 he became instructor to the first battalion of volunteers of the Bas-Rhin. He served with his battalion in 1792. By 1794 he was adjutant-general (with the rank of chef de brigade). After the Battle of Fleurus (1794), in which he greatly distinguished himself for coolness, he was promoted to general of brigade by the representatives on mission. He married Jeanne Louise Elisabeth Berg on 26 April 1796.

For the next five years he was constantly employed in Germany under Jourdan, Moreau, Kléber and Lefebvre, and in 1799 he was promoted general of division and ordered to proceed to Switzerland. It was at this time that he laid the foundations of his military fame, and he particularly distinguished himself in Masséna's great Swiss campaign, and especially at the Second Battle of Zurich. He accompanied Masséna to Genoa, and acted as his principal lieutenant throughout the protracted siege of that city, during which he operated with a detached force without the walls, and after many successful actions he was wounded and taken prisoner at Monte Cretto on 13 April 1800.

The victory of Marengo restored his freedom, and Soult received the command of the southern part of the kingdom of Naples, and in 1802 he was appointed one of the four generals commanding the consular guard. Though he was one of those generals who had served under Moreau, and who therefore, as a rule, disliked and despised Napoleon Bonaparte, Soult had the wisdom to show his devotion to the ruling power; in consequence he was in August 1803 appointed to the command-in-chief of the camp of Boulogne, and in May 1804 he was made one of the first marshals of the Empire. He commanded a corps in the advance on Ulm, and at Austerlitz he led the decisive attack on the allied centre.

Soult played a great part in all the famous battles of the Grande Armée, including the Battle of Jena in 1806. However, he missed the Battle of Friedland because on that day he forced his way into Königsberg. After the conclusion of the Peace of Tilsit, he returned to France and was created (1808) duke of Dalmatia. The award of this title greatly displeased him, for he felt that his proper title would be duke of Austerlitz, a title Napoleon had reserved for himself. In the following year he was appointed to the command of the II corps of the army with which Napoleon intended to conquer Spain, and after winning the Battle of Gamonal he was detailed by the emperor to pursue Sir John Moore's British army. At the Battle of Corunna, in which the British general was killed, Soult was defeated and the British escaped by sea.

For the next four years Soult remained in Spain, and his military history is that of the Peninsular War. In 1809, he invaded Portugal and took Oporto, but was isolated by General Silveira's strategy of contention. Busying himself with the political settlement of his conquests in the French interests and, as he hoped, for his own ultimate benefit as a possible candidate for the Portuguese throne, he attracted the hatred of Republican officers in his Army. Unable to move, he was eventually driven from Portugal in the Second Battle of Porto by Wellesley, making a painful and almost disastrous retreat over the mountains, pursued by Beresford and Silveira. After the Battle of Talavera (1809) he was made chief-of-staff of the French troops in Spain with extended powers, and on 19 November 1809, won a great victory at the Battle of Ocana.

In 1810 he invaded Andalusia, which he speedily reduced. However, because he turned aside to seize Seville, the capture of Cádiz eluded him. He said, "Give me Seville and I will answer for Cádiz." This led to the prolonged and futile Siege of Cadiz, a strategic disaster for the French. In 1811 he marched north into Extremadura and took Badajoz. When the Anglo-Portuguese army laid siege to the city he marched to its rescue, and fought and nearly won the famous and very bloody Battle of Albuera on 16 May.

In 1812, after the Duke of Wellington's great victory of Salamanca, he was obliged to evacuate Andalusia. In the subsequent Siege of Burgos campaign, Soult was able to drive Wellington's Anglo-Allied army back to Salamanca. There, Soult failed to attack Wellington despite a 80,000 to 65,000 superiority of numbers, and the British army retired to the Portuguese frontier. Soon after, he was recalled from Spain at the request of Joseph Bonaparte, with whom, as with the other marshals, he had always disagreed.

In March 1813 he assumed the command of IV Corps of the Grande Armée and commanded the centre at Lützen and Bautzen, but he was soon sent, with unlimited powers, to the South of France to repair the damage done by the great defeat of Vitoria. It is to Soult's credit that he was able to reorganise the demoralised French forces with a rapidity that even took Wellington by surprise.

Although often found wanting tactically - even some of his own aides queried his inability to amend a plan to take into account altered circumstances on the battlefield - his performance in the closing months of the Peninsular War is the finest proof of his talents as a general. Though repeatedly defeated in these campaigns by the Allies under Wellington, many of his soldiers were raw conscripts, while the Allies could count greater numbers of veterans among their ranks. His last offensives into Spain were turned back by Wellington in the Battle of the Pyrenees (Sorauren) and by Freire's Spaniards at San Marcial. Pursued onto French soil, Soult was maneuvered out of several positions at Nivelle, Nive, and Orthez, before dealing Wellington a final bloody nose at the Battle of Toulouse.

The political career of Marshal Soult was by no means as creditable, and it has been said of him that he had character only in the face of the enemy. After the first abdication of Napoleon (1814) he declared himself a Royalist, received the order of St. Louis, and acted as minister of war from 3 December 1814 to 11 March 1815. When Napoleon returned from Elba, Soult at once declared himself a Bonapartist, was made a peer of France and acted as major-general (chief of staff) to the emperor in the campaign of Waterloo, in which role he distinguished himself far less than he had done as commander of an over-matched army.

At the Second Restoration (1815) he was exiled, but not for long, for in 1819 he was recalled and in 1820 again made a marshal of France. He once more tried to show himself a fervent Royalist and was made a peer in 1827. After the revolution of 1830 he made out that he was a partisan of Louis Philippe, who welcomed his support and revived for him the title of marshal-general (previously only held by Turenne, Claude Louis Hector de Villars and Maurice de Saxe). He served as minister of war from 1830 to 1834, as Prime Minister from 1832 to 1834, as ambassador extraordinary to London for the coronation of Queen Victoria in 1838 - where the Duke of Wellington reputedly caught him by the arm and exclaimed 'I have you at last!', again as Prime Minister from 1839 to 1840 and 1840 to 1847, and again as minister of war from 1840 to 1844. In 1848, when Louis Philippe was overthrown, Soult again declared himself a republican. He died at his castle of Soultberg, near his birthplace.

He published a memoir justifying his adherence to Napoleon during the Hundred Days, and his notes and journals were arranged by his son Napoleon Hector (1801 – 1857), who published the first part (Mémoires du maréchal-général Soult) in 1854. Le Noble's Mémoires sur les operations des Français en Galicie are supposed to have been written from Soult's papers.

Soult was a skillful military strategist. An example was his drive to cut off Wellington's British army from Portugal after Talavera, which nearly succeeded. Though repeatedly defeated by Wellington in 1813-1814, he conducted a clever defense against one of history's greatest commanders. Soult's armies were usually well maintained before going into battle. After Vitoria, he reorganized the demoralized French forces of Joseph Bonaparte into a formidable army in a remarkably short time. An exception to this good logistical record was launching the Battle of the Pyrenees offensive when his soldiers only had four days rations.

Tactically, Soult planned his battles well, but often left too much to his subordinates. Wellington said that Soult "never seems to me to know how to handle troops after the battle had begun". An example is at the Battle of Albuera, where he brilliantly turned Beresford's flank to open the battle. But when he found himself facing unexpected opposition from Spanish and British troops, Soult allowed his generals to adopt a clumsy attack formation, failed to act decisively and was finally beaten. Another example of his strengths and weaknesses can be seen at the Battle of the Nive. Soult recognized Wellington's strategic dilemma and took advantage by launching surprise attacks on both wings of the Anglo-Allied army. But French tactical execution was poor and the British general managed to fend off Soult's blows. Sloppy staff work marred his tenure as Napoleon's chief-of-staff in the Waterloo campaign.

Soult allowed his personal ambitions to distract him from his military duty. Greed and ambition caused him to pass up a golden opportunity to take Cádiz. While occupying northern Portugal, his intrigues earned him the nickname, "King Nicolas." Later, he set himself up as a virtual viceroy of Andalusia, looting 1.5 million francs worth of art. One historian called him "a plunderer in the world class."

He married in 1796 Louise Berg (1771 – 1852) and had 3 children: Napoleon (1802 – 1857), 2nd Duke of Dalmatia, who died without male heir, at which time the title became extinct; Hortense (1804 – 1862); Caroline (1817).