Flahaut was the supposed illegitimate son of Talleyrand, and the father of Charles Auguste Louis Joseph Demorny Duc de Morny (who contracted cholera in 1854 and was saved by homeopathy),
Flahaut was the father in law of Henry Charles Keith Petty Fitzmaurice 5th Marquess of Lansdowne, and he was a friend of Granville Leveson Gower 1st Earl Granville and his wife Harriet Cavendish Granville (the daughter of William Cavendish 5th Duke of Devonshire who married Elizabeth Christiana Hervey Duchess of Devonshire, the presumed mother of Frederick Hervey Foster Quin),
Flahaut was born in Paris, the son of Alexandre Sebastien de Flahaut de La Billarderie, comte de Flahaut de La Billarderie, beheaded at Arras in February 1793, and his wife Adelaide Filleul, afterwards Mme de Souza Botelho. Charles de Flahaut was generally recognized to be the offspring of his mother’s liaison with Talleyrand, with whom he was closely connected throughout his life. His mother took him with her into exile in 1792, and they remained abroad until 1798.He entered the army as a volunteer in 1800, and received his commission after the Battle of Marengo. He became aide de camp to Joachim Napoleon Murat, and was wounded at the Battle of Landbach in 1805.
At Warsaw he met Anne Poniatowska, Countess Potocka, with whom he rapidly became intimate.
After the Battle of Friedland he received the Legion of Honour, and returned to Paris in 1807. He served in Spain in 1808, and then in Germany. Meanwhile the Countess Potocka had established herself in Paris, but Charles de Flahaut had by this time entered in his liaison with Hortense de Beauharnais Queen of Holland. The birth of their son was registered in Paris on 21 October 1811 as Charles Auguste Louis Joseph Demorny Duc de Morny.
Flahaut fought with distinction in the Russian campaign of 1812, and in 1813 became general of brigade, aide de camp to Napoleon Bonaparte, and, after the Battle of Leipzig, general of division.
After Napoleon Bonaparte‘s abdication in 1814 he submitted to the new government, but was placed on the retired list in September. He was assiduous in his attendance on Hortense de Beauharnais Queen of Holland until the Hundred Days brought him into active service again.
A mission to Vienna to secure the return of Marie Louise resulted in failure. He was present at Waterloo, and afterwards sought to place Napoleon II on the throne. He was saved from exile by Talleyrand‘s influence, but was placed under police surveillance.
Presently he elected to retire to Germany, and thence to Great Britain, where he married in Edinburgh on 20 June 1817 Margaret Mercer Elphinstone, daughter of Admiral George Keith Elphinstone 1st Viscount Keith, and after the latter’s death 2nd Baroness Keith in her own right. The French ambassador opposed the marriage, and Flahaut resigned his commission.
His eldest daughter, Emily Jane, married Henry Charles Keith Petty Fitzmaurice 5th Marquess of Lansdowne. His youngest daughter, Georgiana Gabrielle de Flahaut, (d. 16 July 1907), married on 2 February 1871 the Marquis de Lavalette, who died in 1881.
The Flahauts returned to France in 1827, and, in 1830, Louis Philippe gave the count the grade of lieutenant-general and made him a peer of France. He remained intimately associated with Talleyrand‘s policy, and was, for a short time in 1831, ambassador at Berlin. He was afterwards attached to the household of the duke of Orleans, and, in 1841, was sent as ambassador to Vienna, where he remained until 1848, when he was dismissed and retired from the army.
After the coup d’état of 1851, he was again actively employed, and from 1860 to 1862 was ambassador at the court of St James’s. He died in Paris on 1 September 1870.
The comte de Flahaut is perhaps better remembered for his exploits in gallantry, and the elegant manners in which he had been carefully trained by his mother, than for his public services, which were not, however, so inconsiderable as they have sometimes been represented to be.
In 1893, Dr. Flahaut witnessed the homeopathic treatment of diphtheria in his home town of Normandy, and he wrote The Petroleum Treatment of Diphtheria,