First known as the Chevalier de Bissy, Henri began a military career at the age of fifteen, becoming second lieutenant in an infantry regiment.

Count Thiard is known for his firmness in Brittany in the period preceding the French Revolution.

During the election of the deputies to the Estates General, in May 1789, he had to face street fighting inRennes, and only stopped the bloodshed at the risk of his life.

The king's steward, Antoine-François Bertrand de Molleville, and himself were stoned by the crowd and forced to take refuge in the governor's palace.

Having remained loyal to the king, he was wounded during the storming of the Tuileries by the rioters on August 10, 1792.