Once there, and for some time, he was an itinerant trader in Venezuela, New Granada, and Peru, but, being investigated by the Inquisition, he moved to La Plata Colony, in present-day Argentina, where he tried some commercial ventures.

From there, O'Higgins proposed to open easy communication between Chile and Mendoza by a way over the Andes, and, his proposition being accepted, he was employed to supervise the project.

On his first harrowing journey over the Andes mountains separating Argentina and Chile during the winter of 1763–64, O'Higgins conceived the idea of a chain of weatherproof shelters.

By 1766, thanks to O'Higgins' efficient execution of this plan, Chile enjoyed all-year overland postal service with Argentina, which had previously been cut off for several months each winter.

[8] In 1764, John Garland, another Irish engineer at the service of Spain who was military governor of Valdivia, convinced him to move to the neighbouring, and less established, colony of Chile as his assistant.

[9] In 1770 the Governor of Chile appointed him, now in his late forties, captain of a column of cavalry to resist the attacks of the Araucanian Indians, whom he defeated, recovered big swathes of territory that had been lost by the Spaniards and founding the fort of San Carlos in the south of the province of Arauco.

[10] In February 1787, the frigate Astrea under the command of Alessandro Malaspina called at Talcahuano, the port of Concepcion, in the course of a commercial circumnavigation of the world on behalf of the Royal Philippines Company.

Following the Astrea's return to Spain, Malaspina produced, in partnership with José de Bustamante, a proposal for an expedition along the lines set out in O'Higgins' memorandum.

José de Espinoza y Tello, one of the officers of the Malaspina expedition, subsequently confirmed the importance of the information sent by O'Higgins in stimulating the Government to initiate an extensive program of exploration in the Pacific.

[13] In 1784 the Governor of Chiloé, Francisco Hurtado del Pino, and Ambrosio O'Higgins had been ordered to open a route over Huilliche territory between Maullín and Valdivia.

The latter had good relations with the Huilliches of Río Bueno and Ranco because of his support for peaceful contact coupled with a religious mission, and refused to establish any new fort in the territory.

This treaty averted invasion from Chiloé, and gave the Huilliches of Osorno support from the Valdivia authorities against raids by the Aillarehue of Quilacahuín.

Figueroa set fire to Indian houses and croplands near his marching route, and arrested a large number of male Huilliches as suspected rebels.

The whole treaty cost 10.897 pesos, which was, according to Diego Barros Arana, an enormous amount of money, considering the size of Chile's treasury.

When war was declared between Britain and Spain in 1797, O'Higgins took active measures for the defense of the coast, strengthening the fortifications of Callao and constructing a fort in Pisco.

He projected and constructed a new carriage-road from Lima to Callao, and his principal attention during his short administration was directed to the improvement of means of communication.

Though Ambrosio O'Higgins never saw or officially recognised his son as his legal heir, he paid for his education in England and left him a portion of his possessions in Peru and Chile.