Born in Borgo San Lorenzo, at the time in the Grand Duchy of Tuscany, he was the son of Francesco, Imperial Count and Florentine Patrician, titles which he would later inherit, and Maria Genta. His father had an active role in the Risorgimento, fought in the battle of Curtatone in 1848, and later become the first mayor of Borgo San Lorenzo of the newly founded Kingdom of Italy, in 1861. Guglielmo was the elder of three sons and a daughter: Alessandro, the only one who left male issues, Galeazzo, Gisella and Alfredo. Though he married twice, he died without issues, and the titles passed to his brother's heirs.