After Mateo de Toro y Zambrano took over as Royal Governor in 1810, he was convinced to call an open meeting of the leading citizens of the city to decide the political future of the colony. He convened such a meeting for the morning of September 18, 1810. The discussion ended with the conformation of the First Government Junta of Chile where Márquez de la Plata was elected as one of the members. After the death of Toro y Zambrano, the illness (and posterior death) of the Vice President, Bishop José Martínez de Aldunate, and the disgrace of their appointed successor Juan Martinez de Rozas as a consequence of the Figueroa mutiny, which Márquez de la Plata had to put down, he was elected president, a position he held between April 2 and July 4, 1811. In September 1811 he was named a member of the New Appeals Court ("Tribunal de Apelaciones"), the body that replaced the colonial Appeals Court ("Real Audiencia") and became its doyen.