In 1790 he became procureur of the Bordeaux Commune, and in July 1791 was elected by the newly created département of the Gironde a member of the court of appeal.

As rapporteur of the diplomatic committee, in which he supported the policy of Jacques Pierre Brissot, he proposed two of the most revolutionary measures passed by the Assembly: the decree of accusation against the King Louis XVI's brothers (the Comte de Provence and the Comte d'Artois) on 1 January 1792, and the declaration of war against the Habsburg ruler Francis II (20 April 1792).

[1] Elected to the National Convention, where he was regarded as one of the most brilliant of the group of orators from the Gironde (although he always read his speeches), Gensonné denounced, on 24 October, the actions of the Paris Commune following the September Massacres.

At the king's trial in late December, he supported an appeal to the people, but voted for the death sentence.

[citation needed] As a member of the Committee of General Defence, and as president of the convention (7 March–21 March 1793), he shared in the harsh attacks of the Girondists on The Mountain.