After extensive experience as an Ambassador, Grimaldi served as Chief Minister of Spain between 1763 and 1778 helping to rebuild Spanish power following its defeat during the Seven Years' War.

For his services as Secretary of State, he was granted the title of Duke of Grimaldi by King Charles III of Spain.

[citation needed] In the service of Spanish Kings Ferdinand VI and Charles III, Grimaldi was minister plenipotentiary in Sweden and Parma, and ambassador to the States-General of the United Provinces.

Charles III named him ambassador to Paris, where together with French Secretary of State Étienne François, duc de Choiseul he negotiated the third Family Compact between France and Spain.

In September 1763, after the dismissal of Ricardo Wall he was named Spanish Minister of State, a position he held until 1776.