He is the grandson (on the paternal line) of Jacques Isnard, merchant curator, was lord of Deux-Frères and Esclapon, and of Claire Courmes, both from old families of the bourgeoisie of Grasse.

On 9 September 1791, he was elected member of Legislative Assembly by the department of Var, in southeastern of France (district of Draguignan.

Attacking the court, and the Austrian committee in the Tuileries, he demanded the disbandment of the king's bodyguard, and reproached Louis XVI for infidelity to the constitution.

Elected to the National Convention in September 1792, he was sent to the army of the North, near Nice, to justify the insurrection ; he announced the take of Sospel and went back to Paris in autumn.

Isnard was presiding at the Convention when a deputation of the commune of Paris came to demand that Jacques René Hébert should be set at liberty, and he made the famous reply: "If by these insurrections, continually renewed, it should happen that the principle of national representation should suffer, I declare to you in the name of France that soon people will search the banks of the Seine to see if Paris has ever existed"[4] On 2 June 1793 he offered his resignation as representative of the people, but was not comprised in the decree by which the Convention determined upon the arrest of twenty-nine Girondists.

On 13 October 1795, now regarded as a royalist, he was elected deputy for the Var to the Council of Five Hundred, where he played a very insignificant role.

Upon the restoration he professed such royalist sentiments that he was not disturbed, in spite of the 1816 law proscribing members of the Convention who had voted to execute the king.