He left Tuscany in October 1579 to bring Italian troops to Spain and lead them during the mission to Portugal.

He came to Italy in July 1584 to ask his brothers to cover his debts, incurring their disapproval for living with a woman with a questionable reputation.

Pietro continued to see his lover Antonia de Carvajal who gave him five children out of wedlock.

He kept writing to his brother Ferdinando I de' Medici, asking for a portion of the family fortune to cover his debts.

Pietro was buried in the Monasterio de la Santissima Trinidad in Madrid.