[6] Hernandarias was elected unanimously by the caudillo in Asunción as the governor of Rio de la Plata province, including Buenos Aires.

These included the creation of the first primary schools, kilns for creating bricks and tiles to replace adobe as a construction material, and the rebuilding of a fortress to protect the city from pirates.

Following the capture of two anchored ships by English privateers on 18 March 1607, he ordered the construction of a larger fort at the mouth of the Matanza River, in what is now the neighborhood of Vuelta de Rocha.

During his term as governor of Buenos Aires, Hernandarias started several expeditions, including ones to Uruguay and Brazil to rein in the Portuguese bandeirantes, explore Patagonia, survey the navigability of rivers and to find the mythical City of the Caesars.

In 1611, visiting judge Francisco de Alfaro ordered the emancipation of all natives working on encomiendas who had been converted by the Jesuits.

[3] Working with carpenter Pascual Ramírez, Hernandarias secured a supply of lumber from Paraguay as well as labor from Spanish colonists and converted natives.

[8][9] In his personal life, he was married to Jerónima de Contreras[2] with whom he had three daughters: Gerónima, Isabel and María[1] Hernandarias retired with his wife to Santa Fe, Argentina, where he died in 1634 at the age of 72.

On his return to Buenos Aires he reported to King Felipe III of Spain describing those lands as excellent for cattle raising, and suggesting they be populated.

In 1611 Hernandarias disembarked fifty heifers and some bulls in Lobos Island and reached an agreement with the natives to protect animals with calves.

This act originated a great cattle herd in the region, the exploitation of which was determinant in the historical processes lending to colonization of present day Uruguay, so far overlooked by Spaniards.