At the resignation of the count of Saint Germain, thanks to the influence of Madame of Maurepas, Montbarrey was appointed in 1778 as Secretary of State for war.

The prince of Montbarrey was an opportunistic and incompetent minister, without morality[4] who involved more energy in his interests and lust with many mistresses[5] than to conduct his department.

[7] After his forced resignation, he moved with his wife and his daughter, the princess of Nassau-Saarbrücken to the Arsenal near the Bastille, in a luxurious building given by the king with a considerable pension.

At the Revolution the furniture, library, gallery of paintings and art objects that decorated the residence of the prince of Montbarrey were seized as property of emigrants and sold to Lord Chattam (eldest son of British Prime Minister William Pitt) who took them to England.

In 1791 he emigrated with his wife in Switzerland in Neuchâtel and in the villages of Cressier and Landeron[9] (when they cross the border they had been robbed of all the money and jewelry they had with them[10]).

Maximilienne of Saint-Mauris-Montbarrey, princess of Nassau-Saarbrücken, daughter of the prince of Montbarrey, died on 2 February 1838 in Maisons-Alfort (near Paris), aged 79 years old.