At the time of the Roman conquest in 222 BC, Mediolanum, as it was then called, was already one of the most powerful cities of the region known as Cisalpine Gaul (on the Roman side of the Alps). Under the emperor Augustus, it became a part of the 11th region of Italy and acquired increasing prestige and economic power until it became the second city of the Western Roman Empire, behind Rome itself. In the 3rd century AD, following the partition of the empire instituted by the emperor Diocletian, it was assigned as residence and main administrative centre for the emperor in the West. The emperor Constantine I (the Great) declared it the seat of the vicar of Italy. In the year 452 Attila the Hun devastated the city, and in 539 the Goths destroyed it.

The city, however, did not entirely perish as a result of these barbarian incursions, and by the second half of the 10th century city life was surging with renewed vigour. Under the Carolingians (the region was incorporated into the dominions of Charlemagne in 774), life in Milan showed increased vitality, particularly through the efforts of the archbishop Ansperto da Biassono, who rebuilt and strengthened the fallen walls of the city in the late 9th century. Under Ariberto da Antimiano (1018–45), the political power of the archbishopric reached its apogee. This assumption of temporal power by the archbishops, dating from about 1000, can be considered as the origin of the subsequent greatness of Milan.

Milan entered the 21st century with a dynamic economy that had transformed itself into a postindustrial powerhouse, thanks to the city’s central market location, a capable commercial and financial class, and the relatively cheap labour of hundreds of thousands of immigrants. Many areas within the city were redeveloped and revitalized, and Milan could justly claim to be a world capital of fashion, design, finance, business services, and media and publishing.