From Madrid he was suddenly summoned to the governorship of Brittany, and in 1787 was appointed by the king to succeed Vergennes in the ministry of foreign affairs.

The comte de la Marck was trying to bring Mirabeau into touch with the court, and for this purpose it was important to secure the assistance of Montmorin.

[1] The death of Mirabeau in April 1791 was a severe blow to Montmorin, the difficulty of whose position was enormously increased after the flight of the royal family to Varennes, to which he was not privy.

He was forced to resign office, but still continued to advise Louis, and was one of the inner circle of the king's friends, called by the revolutionists "the Austrian Committee."

He took refuge in the house of a washerwoman, but was discovered, taken before the Legislative Assembly, and imprisoned in the Prison de l'Abbaye, where he perished in the September massacres.