Born in La Basse-Chapelière, near Maumusson, in what would become the Department of Loire-Atlantique, he was educated at the Collège de Châtillon, in Châteaubriant, before studying law at Rennes.

For acting against the ascendent faction he was declared "traître à la patrie" (Traitor to the Fatherland), and eventually was forced to go into hiding to escape the arrest decreed by the convention (3 October 1793).

[1] Defermon returned to exercising his duties as deputy in December 1794 as supporters of the Girondists are rehabilitated by an increasingly conservative Convention.

He was elected (14 October 1795) to the Corps législatif by the Département of Ille-et-Vilaine and subsequently selected to sit in the Council of Five Hundred (1795–1797), serving as president from 21 May 1796 to 19 June 1796).

[1] Defermon supported the Coup of 18 Brumaire (10 November 1799) of Napoleon Bonaparte and was appointed a member of the Tribunat (25 December 1799) and counselor of state.