The name coleus (testicle) dates back to 1123, adopted by Gisalbertus Attonis known as “Il Colione”, son of the notary Attone (1044–1081), who was in turn son of Ghilberto mentioned in three parchments, who descended from a branch of the Suardi family and who in 1123 was Consul of Bergamo and is considered by historians to be the founder of the family. Of his five children: Guglielmo was Consul of Justice, Ugolone, Sozzo Consul of Justice in 1161 and protagonist in the battle of the Volpino castle. Of his descendants, we remember Sozzo Colleoni, Podestà of Cremona, whilst his nephew Trussardo was consul and rector in Bergamo throughout 1237–1329, Villano, and Alberto who was Consul of Bergamo from 1159 to 1173, allied with the Bonghi and Rivola families against Emperor Frederick Barbarossa. Another descendant was Isnardo Colleoni, who left Bergamo and became Podestà of Milan in 1338. Both gave rise to the different branches of the family.
The Colleonis were a family of notaries who knew how to maintain good relations with the church in Bergamo and with the community.
When the Dukes of Milan of the house of Visconti conquered Bergamo, they had supporters on the Ghibelline side, such as the Suardi, whilst the Colleoni, of the Guelph faction, had to flee into exile. Amongst these, Galeazzo Colleoni – known as Carpione – took refuge in the upper Val Seriana then attempted a botched insurrection in 1315 with Giorgio Del Zoppo, Belfantino Rivola and the Bonghi, which led them first to be taken prisoner then freed on 7th July 1315 after agreeing to a treaty with Matteo Visconti for a lordship in Bergamo that was limited in time.
Paolo Colleoni conquered the castle of Trezzo in 1404, holding it until he was assassinated by his relatives, possibly on behalf of the Visconti. His son was the most famous member of the family, the mercenary leader Bartolomeo Colleoni, Captain General of the Venetian Republic. He and his daughter Medea († 1470) are buried in the Colleoni Chapel in Bergamo, a significant Renaissance monument. Andrea Verrocchio built an equestrian statue for him in Venice, which is counted amongst the most important sculptures of the Italian Renaissance.
Another member of the family was Margherita Colleoni (1455–1483), first wife of the military commander Gian Giacomo Trivulzio, who is buried in the Trivulzio Chapel of the Basilica of San Nazaro in Brolo in Milan.
Bartolomeo’s nephew, Alessandro Martinengo Colleoni, from whom the Martinengo Colleoni branch began, commissioned Lorenzo Lotto to paint the Martinengo Altarpiece for the Dominican church of Santo Stefano, relocating it following destruction of the church for the construction of the Venetian Walls of Bergamo in the church of San Bartolomeo and Santo Stefano.