He was born at Grenoble (Isre). He studied law, and in 1783 obtained a judgeship at Grenoble. He took part in the struggle between the parlements and the court in 1788, and promoted the meeting of the estates of Dauphin at Vizille (July 20, 1788), which on the eve of the French Revolution created an immense stir. He was secretary of this assembly, and drafted the cahiers ("notebooks") of grievances and remonstrances presented by it to King Louis XVI. Thus brought into prominence, Mounier was unanimously elected deputy of the third estate to the states general of 1789.
There, and in the Constituent Assembly, he was at first an upholder of the new ideas, pronouncing himself in favour of the union of the Third Estate with the two privileged orders, proposing the famous oath of the Tennis Court, assisting in the preparation of the new constitution, and demanding the return of Jacques Necker. On September 28, 1789 he was elected president of the Constituent Assembly. Being unable, however, to approve the proceedings which followed, Mounier withdrew to Dauphin, resigned as deputy, and, becoming suspect, took refuge in Switzerland in 1790.
He returned to France in 1801, was named by Napoleon Bonaparte prefect of the department of Illeet-Vilaine, which he reorganized, and in 1805 was appointed councillor of state. He died in Paris. His principal writings are Considerations sur les gouvernements (1789); Recherches sur les causes qui ont empeche les Français de devenir libres (1792), and De l'influence attribuée aux philosophes, aux francs-maçons et aux illuminés sur la Révolution Française. (1801).