He was born in the illustrious Pignatelli family, connected with the highest Neapolitan and Aragonese nobility.

During his embassy in France, he led the Spanish adhesion to the third Pacte de Famille and built a solid friendship with Étienne François, duc de Choiseul, Prime Minister of King Louis XV of France, who saw in Fuentes a "spirit suitable for business", of "broad genius", as well as a "very honest man and attached to our system."

After the fall of Choiseul in December 1770, Fuentes refused to pay respect to Madame du Barry, the new favorite of the French monarch and a former prostitute.

[1] Fuentes married in 1741 with María Luisa de Gonzaga y Caraciolo, II Duchess of Solferino.

The Duchess Mariana originally was going to marry the eldest son of Fuentes, José María III Duke of Solferino, but he died before the marriage could be concluded.