When the French invaded Italy, he led the faction which openly declared in their favour and came to the attention of Bonaparte, who placed much trust and confidence in him.
He was a strong supporter of the political reform of 1796 and when the Cispadane Republic was formed that year he became part of its executive directory.
He was strongly supported in his work by Bernier, bishop of Orléans, who with Giovanni Battista Caprara co-organised the Concordat between Rome and Italian Republic, signed in Paris on 9 September 1803.
He rented Hôtel de Massa as his Paris residence and held many festivities there, making it a key meeting point for the nobility of the First French Empire and launching the fashion for masked or fancy-dress balls.
After Napoleon's abdication, Marie-Louise of Austria made Marescalchi governor of the Grand Duchy of Parma and Piacenza.