In 1803 he negotiated the Louisiana Purchase treaty by which Louisiana was ceded to the United States, and was rewarded by the First Consul with a gift of 152,000 francs. Loyal to the First Empire, he was made grand officer of the Legion of Honour and a count in 1805, and in 1808 he became president of the "Cour des Comptes". His career as Head of the Treasury ended in 1806. In return for these favours, he heaped praise upon Napoleon; yet, in 1814, he helped to draw up the act of abdication of the emperor, and declared to the "Cour des Comptes", with reference to the invasion of France by the Sixth Coalition:  In June of that year, under the First Restoration, Barbé-Marbois was made Peer of France by King Louis XVIII, and confirmed in his office as president of the "Cour des Comptes". Deprived of his positions by Napoleon during the Hundred Days, he was appointed Minister of Justice under the Duc de Richelieu (August 1815), tried unsuccessfully to gain the confidence of the Ultra-Royalists, and withdrew at the end of nine months (10 May 1816). In 1830, when the July Revolution brought Louis Philippe and the Orléans Monarchy, Barbé-Marbois went, as president of the "Cour des Comptes", to compliment the new king, and was confirmed in his position. He held his office until April 1834. In 1829 he wrote the book "Histoire de la Louisiane et la cession de cette colonie par la France aux Etats-Unis de l'Amérique septentrionale ; précédée d'un discours sur la constitution et le gouvernement des Etats-Unis" ("History of Louisiana and of Its Cession to the United States of Northern America; Preceded by a Discourse on the Constitution and Government of the United States"). He published various texts, including: Written in 1780, while secretary to the French Legation to the US Army: "D'Complot du Benedict Arnold & Sir Henri Clinton contre Eunas` States du America General George Washington" One of the first accounts of Arnold's treason, was not published until 1816.