Belloni was born at Pizzighettone, near Cremona, and made his debut in the road cycling world in the wake of his elder brother Amleto.

As an amateur, in 1914 he won the "Small" Giro di Lombardia and the Coppa del Re, as well as the Italian championship.

Having avoided to be called to the front, Belloni won surprisingly, the overall Giro di Lombardia in 1915 (a feat he repeated in 1918 and 1928) and the Milan–San Remo of 1917.

In the 1920s Belloni constantly rivalled with his friend Costante Girardengo, being almost always defeated and gaining for this reason the nickname of "Eterno secondo" ("Eternal second").

He won a total of 43 races as a professional, including 12 stages at Giro d'Italia.