There he served as Aide-de-Camp to General Jean Baptiste Kléber, who both received head wounds at Alexandria caused by an incendiary-device, 13 March 1801.

De Faÿ was present at the Battle of Austerlitz and served in Germany under General Édouard Jean Baptiste Milhaud, commanding the 22nd Chasseurs-à-Cheval Regiment, part of Marshal Joachim Murat's Reserve Cavalry Corps; shortly afterwards, on 24 December 1805, he was appointed Brigadier-General.

He served at the Fall of Madrid, and at the battles of Uclés, Cuenca, Medellín, Talavera, Ocana, Gebora, Albuquerque, Albuera,[7] Usagre, Elvas, being wounded at Villafranca;[8] one of his dispatches was intercepted and deciphered by the British.

He was wounded at the Battle of Borodino, was noted for his distinguished and gallant service at Mozhaysk leading France's Sacred Squadron,[10] and having survived the retreat, later commanded the I Cavalry Corps at Bautzen, Reichenbach[11] Goldberg, Lutzen,[12] and Dresden.

[13] He lost a leg from the knee, famously stating when his valet wept upon sight of his injury: "What are you crying about, man, you have one less boot to polish".

[15]With the restoration of the Bourbons he swore allegiance to Louis XVIII and his loyalty to the king during the Hundred Days[16] was rewarded by being created a Pair de France, 1814 and advanced as Marquis in 1817.