Beginning of his military career
Garro was born in Mondragón, Guipúzcoa. As a youth, Garro enlisted in the Spanish Army and participated in campaigns in Catalonia and Portugal, until he reached the rank of colonel (maestre de campo, the rank immediately below Captain General) of a tercio. His career in Spain then suffered a setback: according to historian Diego Barros Arana, "Due to a violent altercation with a General who had rank of Grandee in Spain, Garro was subjected to the vengeance of a powerful enemy. Preferring instead to live away from the Court, he requested a post in the Indies and obtained the governorship of the province of Tucumán."