 With the restoration of the Bourbons he swore allegiance to Louis XVIII and his loyalty to the king during the Hundred Days was rewarded by being created a "Pair de France", 1814 and advanced as Marquis in 1817.De Faÿ served in several diplomatic posts, including appointment as Ambassador to London in 1819, where he corresponded with Lord Castlereagh.He was briefly Minister of War (November 1819 – December 1821), and Governor of Les Invalides (December 1821), as well as sitting on the Tribunal which sentenced the French Marshal Prince Michel Ney to death.