In 1756, d'Éon joined the secret network of spies called the ('King's Secret'), employed by King
Louis XV without the knowledge of the government. It sometimes promoted policies that were contrary to official policies and treaties. According to her memoirs, the monarch sent d'Éon with the Chevalier Douglas,
Alexander Peter Mackenzie Douglas, Baron of Kildin, a Scottish Jacobite in French service, on a secret mission to
Russia in order to meet
Empress Elizabeth and conspire with the pro-French faction against the
Habsburg monarchy. This is not confirmed by any contemporary sources. At that time the English and French were at odds, and the English were attempting to deny the French access to the Empress by allowing only women and children to cross the border into Russia. D'Éon later claimed that she had to pass convincingly as a woman or risk being executed by the English upon discovery, and therefore travelled disguised as the lady Lia de Beaumont, serving as a
maid of honour to the Empress. However, there is little or no evidence to support this and it is now commonly accepted to be a story told to demonstrate how d'Éon's identification as female had been of benefit to France in the past. D'Éon returned to France in October 1760, and was granted a pension of 2,000
livres as a reward for her service in Russia. In May 1761, d'Éon became a captain of
dragoons under
Marshal de Broglie and fought in the later stages of the
Seven Years' War. She served at the
Battle of Villinghausen in July 1761, and was wounded at Ultrop. After Empress Elizabeth died in January 1762, d'Éon was considered for further service in Russia, but she instead was appointed secretary to the
Duke of Nevers, awarded 1,000 livres, and sent to London to draft the
peace treaty that formally ended the Seven Years' War. The treaty was signed in Paris on 10 February 1763, and d'Éon was awarded a further 6,000 livres. She received the
Order of Saint-Louis on 30 March 1763, becoming the Chevalier d'Éon. This breach of diplomatic discretion was scandalous to the point of being unheard of, but d'Éon had not yet published everything (the King's secret invasion documents and those relative to the were kept back as "insurance"), so the French government became cautious in its dealings with her, even when she sued Guerchy for attempted murder. D'Éon did not offer any defence when Guerchy countersued for libel. She was declared an
outlaw and went into hiding. However, d'Éon secured the sympathy of the British public: the mob jeered Guerchy, and threw stones at his residence. D'Éon then wrote a book on public administration, , which she published in thirteen volumes in
Amsterdam in 1774. Guerchy was recalled to France, and in July 1766 Louis XV granted d'Éon a pension (possibly a pay-off for d'Éon's silence) and a 12,000-livre annuity, but refused a demand for over 100,000 livres to clear d'Éon's extensive debts. D'Éon continued to work as a spy, but lived in political exile in London. D'Éon's possession of the king's secret letters provided protection against further actions, but d'Éon could not return to France in safety. ==Later life==