The Duomo di Monza was originally built as a chapel at the end of the 6th century by Queen Theodelinda, the wife of King of the Lombards Authari and later of Agilulf.

It was situated near to the royal palace, in an area on the outskirts of the small town of Monza, a short distance from the River Lambro.

The basilica had certainly already been constructed in 603, when the abbot Secundus of Non baptised the heir to the Lombard throne Adaloald there. The basilica had links with both Saint John the Baptist (to whom the queen would very probably have prayed for motherhood) and the papacy in Rome – particularly Pope Gregory I.

The queen played a key role in converting the Lombards from Arian Christianity to Catholicism, a complex process which would only be complete a century later, under the reign of King Liutprand. The Church thus played the role of "shrine" for the Lombard Kingdom and Theodelinda was later honoured with the Theodelinda Chapel, also known as the Zavattari Chapel.

The Treasury holds extraordinary testimony of the first few centuries of the cathedral's life, containing as it does many liturgical artefacts and donations made by the queen, who would be buried in the cathedral after her death. The Treasury also contains gold and ivory donated by King Berengar at the start of the 10th century.

The transformation of the cathedral from the old basilica to the current Duomo came between the 13th and 14th centuries, under the rule of the House of Visconti. It is no coincidence that the crucial year was 1300 – the year of the "great pardon" and the first Jubilee as proclaimed by Pope Boniface VIII.

The architect, Matteo da Campione, certainly lived up to the Visconti's aspirations to build a great basilica for imperial coronations. According to Germanic traditions, emperors had to accept three crowns: the silver crown in Aachen, the golden crown in Rome and the "Iron" Crown in Monza (or Milan).