In November 1560 he was appointed by the governor of Chile, García Hurtado de Mendoza, as capitán general y teniente gobernador para poblar, fundar, repartir tierras y encomendar indios en la provincia de Cuyo (commander-in-chief and plenipotentiary for the province of Cuyo), in what is now western Argentina. Castillo organized and expedition that crossed the Andes, passing through the Aconcagua and Uspallata valleys, reaching the Huentata valley on 22 February.