Charles VIII of France made him Secretary of the Treasury, raised him to the first place in the Council of State, and, according to the historian Francesco Guicciardini, would undertake nothing in the government of his kingdom without the advice of Briçonnet.

Briçonnet, thus won over to the Sforza interest, adroitly encouraged the warlike dispositions of his sovereign, triumphed over the opposition of the royal council, of the Duke of Bourbon, and of Anne of France, the Duke's wife, influenced Charles to sign a secret treaty with Sforza, and assured the king of his ability to raise the funds necessary to carry on the war both on land and sea.

Accompanying Charles on his expedition, he provoked a mutiny in the French army, by his treachery in sacrificing the Pisans, allies of France, to their enemies, the Florentines, and had he not hidden himself from the fury of the soldiers they would have taken his life.

Charles had entered Rome as a conqueror, greatly irritated with Alexander VI for having stirred up opposition to him; but the adroit Briçonnet reconciled his royal master with the pope, and for reward received the cardinal's hat.

Soon after this Briçonnet, induced by a tempting promise of preferment for one of his sons, tried to persuade Charles to break off the peace negotiations and support with an army the Duke of Orleans' claims to the Duchy of Milan.

Charles, however, preferred the counsels of Philippe de Comines and sacrificed the interests of the duke, and the king's premature death put an end to the influence of Briçonnet, Louis XII giving his confidence to the Cardinal d'Amboise.

By his king's direction Briçonnet took steps to assemble at Pisa a council of cardinals opposed to the policy of Julius, and bent on the reformation of the head and hierarchy of the Church.