[2] He was the brother of Louis-Joseph de Montmorency-Laval, Cardinal-Bishop of Metz,[3] who was appointed Grand Chaplain of France in 1786 following the scandal of the Affair of the Diamond Necklace, but had to emigrate in 1791 and died in exile.

[4] Initially known under the title Marquis of Laval,[5] he joined the musketeers during the War of the Austrian Succession on 1 January 1741, serving in the Flanders campaign in 1742.

He was at the Battle of Minden on 1 August 1759, and was appointed Lieutenant-General of the Armies ("Lieutenant Général des armées") on 17 December 1759.

[8] Due to his services to the Crown, he was elevated to the dignity of Hereditary Duke, by letters of October 1758,[1] which was registered on the following 29 November, bringing together the barony of Arnac and other neighboring lands to the marquisate of Magnac, which were combined into the duchy of Laval, for him and his male descendants, with extension to the children and male descendants of his relative, the late Joseph-Pierre, Count of Montmorency-Laval (of the House of Laval-Tartigny), who died at the Battle of Hastenbeck.

As the granddaughter of Charles-Denis de Bullion, Marquis of Gallardon, she was heiress to his lands at Fervaques and Esclimont, which passed into the House of Montmorency.