Born in Saint-Émilion, Gironde, Aquitaine, he had already gained a reputation as a lawyer in Bordeaux by the time of the Revolution.

In 1790 he was made administrator of the Gironde, and in 1791 president of the criminal tribunal, being elected to the Legislative Assembly as one of the group of deputies known subsequently as Girondists.

Nevertheless, he remained a Royalist, and, with Armand Gensonné and Pierre Victurnien Vergniaud, even addressed a letter to the king soliciting a private interview.

In March 1793 he had several meetings with Georges Danton, who was anxious to bring about a rapprochement between the Girondists and The Mountain during the Revolt in the Vendée, but he unconditionally refused to join with the man whom he held responsible for the September Massacres.

Guadet was targeted during the fall of the Girondists, and his arrest being decreed on 2 June 1793, he fled to Caen, and afterwards hid in his father's house in Saint-Émilion.