He was the Royal Governor of the Captaincy General of Chile from December 28, 1755, to September 9, 1761, and Viceroy of Peru from October 12, 1761, to July 17, 1776.

There he served with the contingent under the command of the Carrillo de Albornoz, Duke of Montemar that defeated the Austrians at the Battle of Bitonto in the War of the Polish Succession.

He traveled throughout colonial Chile, and ordered the construction of fortifications on the coast and along the frontier with the Mapuche people (for example, Santa Bárbara).

In Santiago he began important public works and administrative tasks, including improvements to bridges over the Río Mapocho, a market in the Plaza de Armas, and the reform of the Royal University of San Felipe (1757).

On October 12, 1758, he established the first police force in Chile, called the Dragones de la Reina (Dragoons of the Queen).

On October 12, 1761, Amat succeeded José Manso de Velasco, 1st Count of Superunda as Viceroy of Peru.

Tradition says the Paseo de Aguas was built in honor of Amat's mistress, the actress Micaela Villegas, better known as La Perricholi, a Mestiza woman.

It provided the basis for the Prosper Mérimée comic novella Le Carrosse du Saint-Sacrement, which in turn was the basis for both the Jacques Offenbach opéra bouffe La Périchole and the Jean Renoir 1953 film Le Carrosse d'or (The Golden Coach).

[2] In subsequent voyages ordered by Amat, de Bonechea was the first European to explore most of French Polynesia.

The palace still stands on Barcelona's famous La Rambla, where it hosts various temporary art exhibitions and cultural events.