Having already proved himself a worthy entrepreneur under his fathers guidance, Count Fenzi acquired in 1805 the management of Bosi, Mazzarelli & Co., his entrepreneurial sense was rewarded by the economic success of the company.

The Livorno banking house of Senn joined with the Florentine firm of Fenzi to secure the concession for the Strada Ferrata Leopolda, designed to link Leghorn with Florence by way of Empoli, and the line was begun in 1841, to be finished ultimately in June 1848.

By 1845 the desire for a network of railways had led, according to one estimate, to sixteen projects which lay on the grand duke's desk for consideration.

Fenzi was also an investor in the Tuscan steel industry and had owned the Gavorrano mine, the Mammiano ironworks on the Pistoia Apennines, and was a shareholder of the "Società per l'Industria del Ferro".

He had a career as a politician as member of the Tuscan Senate between 1848 and 1849 and was among the biggest supporters of the return of the Grand Duke in Tuscany.