Bianca was born in Pavia as the eldest daughter of Galeazzo Maria Sforza, Duke of Milan, by his second wife, Bona of Savoy, whom he had married in 1468, a year after the death of his first wife, Dorotea Gonzaga, who did not bear him children.
When Bianca was not yet five years old, her father was assassinated inside the Church of Santo Stefano in Milan on 26 December 1476, which was the Feast Day of St. Stephen.