His mother Maria Magdalena of Austria was a sister of Queen Margarita of Spain and Ferdinand II, Holy Roman Emperor.

When his brother was elected Grand Duke, Leopoldo acted as his advisor for manufactures, agriculture and trades.

Leopoldo, a disciple of Galileo, took a real interest in the proceedings of the justly celebrated academy 'Del Cimento' (the test), signing its correspondence, following closely the work of Evangelista Torricelli da Modigliana, inventor of the barometer.

Leopoldo was also a great collector of rare books, paintings (the Venetian collection at the Uffizi is inherited from him), drawings, statues, coins and self-portraits.

On 12 December 1667 Pope Clement IX named him cardinal of Santi Cosma e Damiano.