He was born at Florence, the second son and third child of Cosimo II de' Medici, Grand Duke of Tuscany and Maria Maddalena of Austria.

[3] Upon discovering of the closeness that had formed between the two, the Pope sent Gian Carlo back to Florence, declaring him "too handsome and too young" to be "spiritual advisor" to the Queen.

Here, Giancarlo led a life of dissipation, taking many mistresses; nevertheless, Ferdinando II delegated to him the management of the state's financial affairs.

After one of his mistresses died, "the sexton who buried her said that Cardinal Gian Carlo, remembering the convulsions of that ivory body, came to look once more; and overcome by so much beauty, entered the vault and lay with her for the last time."

He founded the Accademia degli Immobili and contributed to the construction of Teatro della Pergola and the enrichment of the Galleria Palatina di Palazzo Pitti.