Démeunier was a deputy for the Paris "tiers état" at the Estates-General of 1789 preceding the French Revolution. He was elected "administrateur de la ville de Paris" in 1791. He resigned from this post in 1792 and sought refuge in the United States until the end of the Reign of Terror, when he returned to France.
He was a member of the Tribunat, from its creation, and he became its president in 1800.
As a member of the Masonic Lodge Les Neuf Sœurs, Démeunier was particularly active in the management of the newly-created Lycée of which "Monsieur", the Comte de Provence, was the principal Maecenas. This Lycée was created by combining the Musée de Paris with the Musée Scientifique—both had been created by the Société Appolonienne, the origin of which lay with Court de Gébelin who had been the Lodge's secretary in 1779 and was also a Royal Censor. The aim of these institutions was to provide good-quality education to the general public. After the return of Louis XVIII to Paris, the Lycée remained active under the name "Athenée Royal", until 1848.
Jean-Nicolas Démeunier may also be considered one of the key figures in the organisation of support for the American cause, by the Lodge. For example, his
was to be of great influence on the democratic experiments in Belgium in the few years preceding the French Revolution. (For a detailed discussion of events and relevant sources see Gorman 1925, reference below.)
The real significance of "L'Amerique Indépendante..." lay in the fact that it was published as a separate volume of Démeunier's contributions to Charles Joseph Panckoucke's "Encyclopédie méthodique", which had been corrected and debated in correspondence with Thomas Jefferson.
Thomas Jefferson also was the source of "Précis historique de la révolution des Etats Unis d'Amérique, précédé de l'histoire de ses provinces, jusq'à l'époque de la révolution, et suivi du Manifeste ou de l'acte d'Indépendance des treize Etats-Unis", anonymously published in Ghent by Goessin in 1789.