Gian Marias birth was also important in that it helped cement his fathers rule over Milan.

Gian Galeazzo, while admittedly a Visconti - the family who had ruled over Milan since 1277, had gained control over Milan after overthrowing, imprisoning and poisoning his predecessor and uncle, Bernabo Visconti and his two eldest sons.

Following his father's death of plague in 1402, he 14-year old Gian Maria assumed the title of duke at the age of thirteen, under his mother's regency.

The Duchy of Milan soon disintegrated: among the various parties contending its lands, the condottiero Facino Cane prevailed.

A plot by a party of Milanese Ghibellines was raised against the Duke when Facino Cane was terminally ill in Pavia, and Gian Maria was assassinated in front of the church of San Gottardo in Milan.