On 21 October 1817, Monterroso married Juan Antonio Lavalleja, then still a divisional commander under Colonel Fructuoso Rivera, in the Uruguayan village of Florida. They were wed by proxy, as Lavalleja was at the time leading troops against the Portuguese in the Portuguese conquest of the Banda Oriental. the Lavallejas were captured by the Portuguese under Carlos Frederico Lecor in Salto and imprisoned in Rio de Janeiro until the end of the war. In this time, Monterroso involved herself in the politics of the time, delivering and managing Uruguayan correspondence in resistance to the Portuguese.