Tronchet had an interesting relationship with Louis XVI. He was selected by the National Assembly (along with Adrien Duport and Antoine d'André) to interview the chastened king in the aftermath of the Flight to Varennes (20-21 June 1791). Eighteen months later, Tronchet was chosen by King Louis XVI as his defense counsel at his trial, and performed this difficult and dangerous task with ability and courage.
During the period of the Directory, he was a deputy at the Council of the Ancients, where he unsuccessfully opposed the resolution that judges be nominated by the executive directory. Under the Consulate he was the president of the tribunal of cassation, and collaborated in preparing the final scheme for the civil code. He had a marked influence on the code, and succeeded in introducing common law principles in spite of the opposition of his colleagues, who were deeply imbued with Roman law. Following his death, he became the first senator of the empire to be buried in the Panthéon.