The Chevalier d'Eon is best known for his remarkable experiments with gender. He spent the first fifty years of his life as a man, acting as a diplomat and secretly as a spy for the King of France, in St Petersburg and London. Then, following increasing rumours from 1771 onward that he was in fact a woman disguised as a man (which he did nothing to quash and may even have encouraged), he was publicly announced as a woman by the King of France and ruled a woman by the court of King's Bench in London. (In London, the announcement was necessary because increasingly frenzied gambling had been taking place on the question of d'Eon's sex and a judgement of some sort was required, since d'Eon himself showed no sign of answering the issue one way or another.)
Following these judgements, d'Eon returned to France in 1777, where on the order of Louis XVI she (the change of pronoun is deliberate) began wearing women's costume. She remained in some seclusion until 1785, when she returned to England and spent her remaining years in London, making a living by giving fencing demonstrations in female costume. Following the French Revolution, her remaining pension from the French king dried up and in her final years she lived in straitened circumstances. Following her death in May 1810, her housemate of fifteen years came to lay out her body and discovered, in stark contrast to everything she had believed, that d'Eon was biologically a man.
D'Eon was the author of several works, including an unpublished autobiography (finally published by Johns Hopkins Press in 2000) and a collection of lives of female saints of the early church who had lived as men. The decision to live as a woman seems to have been prompted not by a feeling of being a woman in a man's body, but as a political necessity (imposed by others). It was also, in part, a spiritual decision, for d'Eon's devout Christianity in later life seems to have been deeply intertwined with a conviction that women could more easily live a spiritual life.