He backed French Republican ideals after Napoleon Bonaparte's invasion of Italy and occupation of Lombardy, renouncing his noble titles of count of Castiglione Lodigiano, Grandee of Spain, Duke of San Gabrio, feudal lord of Romagnano, marquis of Incisa, co-lord of Castelnuovo Belbo, and feudal lord of Gorgonzola, Camporicco and Cassina de' Pecchi.
In his Storia dell'invasione dei francesi repubblicani nel Milanese, Pietro Verri criticised the choice of men for this diplomatic mission and their lack of preparation, reporting Serbelloni's speech to Milan's council as including the phrase "Citizens, either my bones will remain in Paris or I shall bring you back liberty".
He took part in the Cisalpine Republic's foundation and became president of its first-ever executive Directory from 29 June 1797.
On 3 frimaire in Year VIII (24 November 1799) Franchi (who had fled to Chambéry) ordered Serbelloni to congratulate the Consuls of France.
[2] Serbelloni accompanied Napoleon back to Italy during the 1799–1800 campaign, returning to Milan after the victory at the battle of Marengo to bring order to his lands and estates which were seized by the Austrian-Russian troops of the Second Coalition when it occupied the city from 29 April onwards.