Mousson was the descendant of Huguenot refugees from Le Mas-d'Azil in the French County of Foix who took up refuge on the Vaud side of Lake Geneva, in Morges following the revocation of the Edict of Nantes by Louis XIV. His father was a reformed pastor. Mousson studied law at the Academy in Lausanne. He continued his studies at the University of Tübingen, where his obtained his doctorate in 1796. After the French invasion of Switzerland and the establishment of the Helvetic Republic, he took in January 1798 the position of deputy of Bursins in provisional meeting of the "Lemanischen Republic" in part and was its secretary.