He was educated at Limoges, and entered the cavalry school at Saumur in 1826, becoming maréchal des logis in the 4th Hussars two years later. The share taken by his regiment in supporting the revolution of 1830 was regarded as insubordination, and next year Fialin was dismissed from the army. He became a journalist, and in 1833 became a strong Bonapartist, assuming the title of comte de Persigny, said to be dormant in his family.
He planned the attempt on Strassburg in 1836 and that on Boulogne in 1840. At Boulogne he was arrested and condemned to twenty years' imprisonment in a fortress, shortly afterwards commuted into mild detention at Versailles, where he wrote a book to prove that the Pyramids were built to prevent the Nile from silting up. This was published in 1845 under the title, De la Destination et de l'utilité permanente des Pyramides.
At the revolution of 1848 he was arrested by the provisional government, and on his release took a prominent part in securing the election of Louis Napoleon to the presidency. With Morny and the marshal Saint Arnaud he plotted the restoration of the empire, and was a devoted servant of Napoleon III. He succeeded Morny as minister of the interior in January 1852, and later in the year became senator. He resigned office in 1854, being appointed next year to the London embassy, which he occupied with a short interval (1858-1859) until 1860, when he resumed the portfolio of the interior. But the growing influence of his rival Rouher provoked his resignation in 1863, when he received the title of duke.
A more dangerous enemy than Rouher was the empress Eugenie, whose marriage he had opposed and whose presence in the council chamber he deprecated in a memorandum which fell into the empress's hands. He sought in vain to see Napoleon before he started to take over the command in 1870, and the breach was further widened when master and servant were in exile. Persigny returned to France in 1871, and died at Nice on the 11th of January 1872.
See Mémoires du duc de Persigny (2nd ed., 1896), edited by H de Laire d’Espagny, his former secretary; an eulogistic life, Le Duc de Persigny (1865), by Delaroa; and Emile Ollivier's Empire liberal (1895).