It is one of the oldest churches in Milan. The construction began outside of Porta Vercellina in 379 AD. It was a small church with a nave and two aisles. After 7 years it was consecrated by Bishop Saint Ambrose who after his death became Milan's beloved patron saint. He was buried here in 397 AD.
During the 8th century the Benedicts, with the approbation of the archbishop Pietro, built a convent adjacent to the basilica.
The Campanile dei Monaci, the bell tower, was built between 822 and 859. By the 10th century with the growth in population the basilica became inadequate in size thus the enlarging and restructuring began. The new apse, the new and higher tambour and the left bell tower in Lombard Romanesque style know as the Campanile dei Canonici were built. The construction of the basilica completed in 1889.
Portico della Canonica was built by the commission from Cardinal Ascanio Sforza.
The basilica was damaged in the World War II and restored in 1955 by Ferdinando Reggiori.
In 1857 Archduke Maximilian of Hapsburg restored the original plain lines of the basilica in Neoclassical and Baroque style.
On the walls under the porticoes there are bas-reliefs, pagan and early Christian inscriptions, memorial stones and the Sarcophagus of Archbishop Anspert who died in 881.
The original wooden doors of the basilica with panels of scenes from the Lives of David and Saul are preserved in the Basilica's Museum. The ones that are used on the portal are the copies.
Inside the basilica at the end of the right aisle, there's an access to the Sacellum of San Vittore in Ciel d'Oro which was built during the 4th century.
