The advancement was secured by a playoff victory over Pisa with 4-3.
Reaching Serie A was former Prime Minister Berlusconi's goal with the club he bought in 2018, a year after selling Milan, which he turned into a big club during his 30 years as main owner.
Monza was still in Serie C when Berlusconi acquired the club in 2018 for, according to reports, the equivalent of SEK 30 million.
The team advanced to Serie B 2020, but failed to take another step despite prestigious signings such as Kevin-Prince Boateng and Mario Balotelli.
In the last season, they have instead relied on experienced Serie B players.
Adriano Galliani, Berlusconi's right hand man in Milan, controls Monza.
He made headlines in January 2019 when he recruited 30 players.
During Berlusconi's and Galliani's time in Milan, they won eight Serie A titles and won the European Cup five times.