In 1787 he was nominated as president of the Assembly of Notables, in which capacity he attacked the fiscal policy of Calonne, whom he succeeded as head of the conseil des finances on May 1 1787. Once in power, he succeeded in making the parlement register edicts dealing with internal free trade, the establishment of provincial assemblies and the redemption of the corvee; on their refusal to register edicts on the stamp duty and the proposed new general land-tax, he persuaded King Louis XVI to hold a lit de justice, to enforce their registration. To crush the opposition to these measures, he persuaded the king to exile the parlement to Troyes (August 18, 1787). On the agreement of the parlement to prolong the direct tax on all kinds of income, he recalled the councillors to Paris. A further attempt to force the parlement to register an edict for raising a loan of 120 million livres met with determined opposition. The struggle of the parlement against the incapacity of Brienne ended on May 8 in its consenting to an edict for its own abolition, with the proviso that the states-general should be summoned to remedy the disorders of the state.
Brienne, who had in the meantime been made Archbishop of Sens, now faced almost universal opposition; he was forced to suspend the Cour plenière which had been set up to take the place of the parlement, and to promise that the states-general should be summoned. Even these concessions were not enough to keep him in power, and on August 29 he had to retire, leaving the treasury empty. On December 15 following, he was made a cardinal, and went to Italy, where he spent two years. After the outbreak of the French Revolution he returned to France, and took the oath of the Civil Constitution of the Clergy in 1790. He was repudiated by the pope, and in 1791 had to give up the biretta at the command of Pope Pius VI.
He retired to an abbey confiscated in the revolution. He repudiated Catholicism in 1793, at the height of the French Revolution.