In 1756, he commanded the expeditionary force sent to St Lucia, and the Carenage quarter of the island was renamed Castries after him.

Next, he was made capitaine lieutenant of the company of Gendarmes écossais and commandant of the Gendarmerie from 1770 until his retirement in 1788.

de Castries deeply studied the dossiers sent him, and was highly energetic in these roles - hence his saying "I would like to sleep more quickly" ("Je voudrais dormir plus vite").

In politics, nevertheless, his views were rather conservative, if one judges by his "Réflexions sur l'esprit public", addressed to the King in 1785 - for him, monarchy's difficulties were summed up as a problem of authority; it was enough to show firmness and everything would be back in order.

In 1792, at the time of the Prussian invasion of the Champagne, he and the marshal of Broglie commanded a corps in the princes' army.

He died in 1801 at the home of his old enemy Charles William Ferdinand, Duke of Brunswick, now his friend.