Diplomat, writer, spy, and Freemason, a member of the elite Dragoons and one of the best swordsmen France, whose true gender was a source of speculation and provoked public bets in the late 18th century. Generally it was believed that d'Eon was born female, but he had started to dress as a man in his childhood, and changed back from "a bad boy into a good girl" when his secret was revealed decades later. After his death it turned out that he was a man who had dressed as a woman. D'Eon is often called the patron saint of transvestites. Voltaire once famously called him "A nice problem for history."

"Let us then live the life of children of God, and let us stop simply usurping that name. God makes clear whenever he pleases that he is the master of heart. He alone can change us;he alone must be glorified for our change; for allowing us to overcome prejudices from birth and habit." (from 'Extrait de l'Epître de la Chev. d'Eon à Madame la Duchesse de Montmorency-Bouteville à Versailles mai 1778', from Monsieur d'Eon is a Woman by Gary Kates, 1995)

Charles-Geneviève-Louis-Auguste-André-Thimothée d'Eon de Beaumont was born in Tonnere, into a family of lawyers. His father, Louis d’Eon de Beaumont, was an attorney and Sub-Delegate of the Paris Intendancy; his mother, lady Françoise de Chavanson, was a noblewoman from an old and wealthy family. Little is known of d'Eon's childhood and his book of memoirs, La Vie Militaire, politique, et privée de Mademoiselle d'Eon (1779), written by his friend La Fortelle, is not very reliable. D'Eon tells in it that he was born female, and raised from the start as a boy. His father Louis was in debts and he would get a large inheritance from Françoise's family only if she had a son. D'Eon also reveals, that until the age of ten, he was "under the yoke of an involuntary urinary flow."

D'Eon learned to read at a very young age, and at school he excelled in languages and won awards for memorization. After graduating from College Mazarin in Paris in 1749 he worked as the secretary of Monsieur de Sauvigny, administrator of the fiscal department of Paris. There he completed his first book on French government finance. He then worked as a royal censor, spending his time with books. In 1756 d'Eon joined the King's Secret, a network of spies, who worked more for the King himself than the Foreign Ministry.

To create contacts between Russian and France, Louis XV sent d'Eon in 1756 on a secret mission to Russia to meet Empress Elizabeth I. On the journey d'Eon served as a secretary of the Chevalier Douglas, but disguising himself as a woman, he could in secret start negotiations with Elizabeth. His journey was successful. By 1757 Russia and France had reestablished diplomatic relations.

D'Eon returned to France in 1761. Next year he was appointed a captain of the elite Dragoons. The Seven Years' War, which had started in 1756, was nearing its end, and d'Eon fought only in one major campaign. However, he showed courage and skill, and also was wounded in the head and thigh. For his services and help in Franco-Russian peace negotiations, d'Eon was awarded at the age of thirty-five the rare Cross of Saint-Louis. The Cross raised him in noble rank – he was known as the Chevalier d'Eon.

In 1763 d'Eon was appointed Plenipotentiary Minister in London. At the same time he worked as a spy for the King, who played with an invasion plan. D'Eon had reached the peak of his career and fall followed soon. His lifestyle was sumptuous, he enjoyed fully his high position, and he spent much money to create right connections with English counts, dukes, and lords. From wineyeards in his own estate or neighboring districs, he imported and gave as gifts Burgundian wine. This generosity made him very popular among aristocratic wine connoisseurs.

When he was informed that he would have to give up the title of Plenipotentiary Minister, he complained of his treatment to a number of influential friends. Nobody helped him. In October 1763 he refused to follow orders to leave England. D'Eon was afraid that he would be kidnapped and taken to France and in a letter to Louis XV he claimed the new ambassador, the Comte de Guerchy tried to have him poisoned. "Subsequently I have discovered that M. de Guerchy caused opium, if nothing worse, to be put in my wine, calculating that after dinner I should fall into a heavy sleep, that they would put me, still asleep, onto a couch and, instead of my being carried home, I should be carried down to the Thames where probably there was a boat waiting ready to abduct me."

The book which made d'Eon famous was Lettres, mémoires, et négociations particuliéres (1764), a secret diplomatic correspondence dealing with his recall. Noteworthy, he did not publish the King's secret plan to invade England, which perhaps saved his life. In 1766 Louis XV granted him an annual pension of 12,000 livres for the services he had rendered to the King in Russia, in the army and other commissions. D'Eon continued to work as a spy, but he had also another hobby – books. During his London residency he spent much of his time in his library, which contained eventually some 6,000 volumes and 500 rare manuscripts.

D'Eon's political exile lasted fourteen years. In 1770 rumors started to spread that the Burgundian Chevalier was a woman, and soon people began to bet on the question. D'Eon's behavior was not considered effeminate, he looked like a man, and used his uniform of a French Dragoon captain. On the other hand, the rumors did not help d'Eon's work in the King's Service.

After Louis XV's death in 1774, D'Eon negotiated with Pierre Augustin Caron de Beaumarchais (1732-1799), famous for his play The Barber of Seville (1775) and later The Marriage of Figaro (1784), about his return to France. Louis XVI, the new King, rejected the old plans for an invasion of Britain, but he was much interested in getting back d'Eon's secret papers. One of d'Eon's demands in the negotiations was that the government publicly recognizes him as a woman – the King had no objections, and ordered him to wear women's clothes. He was also granted funds for his female wardrobe. In pictures d'Eon is often depicted in relatively comfortable clothes, skirts, and low heels instead of high heels of women. On his left breast d'Eon carried the Cross of Saint-Louis.

D'Eon returned to France in 1777 and met in November Louis XVI and Maria-Antoinette. Next year, when France joined the American War of Independence against Britain, d'Eon sent letters to King's ministers, begging them to continue his military service and send him to America. Instead he was taken to a dungeon beneath the Château of Dijon, where he spent nineteen days. The following six years he lived mostly with his mother at the family's home in Tonnerre. Christian faith, especially Jansenist ideas, became important in his life

In 1785, four years before the French Revolution, d'Eon returned to England where he lived the rest of his life. England was for him "a country more free than Holland and well worthy of being visited by a man of thought and lover of liberty..." Horace Walpole met d'Eon in 1786 and found him loud, noisy, and vulgar – "her hands and arms seem not to have participated of the change of sexes, but are fitter to carry a chair than a fan." James Boswell wrote that "she appeared to me a man in woman's clothes." After the Revolution d'Eon's annual pension was suspended, and he had to sell his huge library to cover part of his debts. To earn money he participated in fencing tournaments, fighting once with the Price of Wales. In 1796 he was wounded seriously. During his last years he lived in misery, sharing his apartment for about fourteen years with a widow, Mrs. Cole, whose husband had been an admiral. In 1792 he sent a letter to France to the National Assembly, and volunteered "to fight and die for the nation, the Law, and the King" and to lead a division of women soldiers against Austria. In 1805 he signed a contract for his autobiography, entitled La Pucelle de Tonnerre, which was never published.

D'Eon died in London on May 21, 1810. Postmortem examination proved that he was not a hermaphrodite, or a woman, but a male. A group of expets, who were called to the apartment, performed "a complete inspection and dissection of the sexual parts" of d'Eon's body. The physicians also discovered that his Adam's apple did not protrude and he did not have femalelike breasts. It has been suggested that d'Eon had Hypogonadotropin eunuchoidism or Kallman's syndrome, a congenital sexual disorder characterized by underdeveloped genitalia and sterile gonads.

D'Eon had been a spy in international politics but the rest of his life he spent as a spy in the world of women, dying peacefully during his self-created mission. Perhaps to protect the reputation of his lady friends who trusted him, he did not reveal his true gender. It is noteworthy, that d'Eon lived through the period, during which Voltaire (1694-1778) questioned the doctrines of the Church, Rousseau (1712-1788) declared "Man is born free; and everywhere he is in chains," the ancien régime was destroyed in the French Revolution, and Napoleonic Wars spread the ideals of liberty, equality, and fraternity. Although d'Eon did not share the views of the later revolutionary leaders, Marat, Danton or Robespierre, he greeted with enthusiasm the storming of the Bastille and its message of freedom. He admired the patron saint of Amazonian women, Joan of Arc, but when she equipped herself with armour, d'Eon wore a dress and and cap as a challenge to traditional gender roles.

For further reading: La Vie Militaire, politique, et privée de Mademoiselle d'Eon by La Fortelle (1779); The True Story of the Chevalier d'Eon by E.A. Vizetelly (1895); D'Eon de Beaumont: His Life and Times by Octave Homberg (1911); La vie étrange de la chevalière d'Eon by Armand Charmain (1929); The Chevalier d'Eon 1728-1810 by Marjorie Coryn (1932); L'Estrange Destinée du chevalier d'Eon 1728-1810 by Pierre Pinsseau (1945); D'Eon: chevalier et chevalière by André Frank (1953); Royal Spy: The Strange Case of the Chevalier d'Eon by Edna Nixon (1965); The Enigma of the Age: The Strange Story of the Chevalier d'Eon by Cynthia Cox (1966); Le Chevalier d'Eon et son problème psychosexuel by M. Cadéac (1966); Monsieur d'Eon Is a Woman by Gary Kates (1995); Monsieur d'Eon by Mark Brownell (2001) 

Selected works:

• Essai historique sur les differentes situations de la France par rapport aux finances sous le regne de Louis XIV et la regence du duc d'Orléans, 1753
• Mémoires pour servir à l'histoire générale des finances, 1758 (2 vols.)
• Note remise à son excellence Claude-Louis-François-Regnier, comte de Guerchy..., 1763
• Lettres, mémoires, et négociations particuliéres, 1764
• Nouvelles lettres du chevalier d'Eon, 1764
• Dernière lettre du chevalier d'Eon de Beaumont à M. le comte de Guerchy, en date du 5 août 1767, 1767
• Les Loisirs du chevalier d'Eon de Beaumont..., 1774 (13 vols.)
• Pièces rélatives aux démélés entre Mademoiselle d'Eon de Beaumont... et le Sieur Caron, dit de Beaumarchais, 1778
• Très-humble réponse à... Beaumarchais, 1778
• Réponse de Mademoiselle d'Eon à Monsieur de Beaumarchais, 1778
• Epître aux Anglais dans leurs tristes circontances présentes, 1788
• The Maiden of Tonnerre: The Vicissitudes of the Chevalier and the Chevaliere D'Eon, 2001