Casares became a vocal opponent of Buenos Aires Province Governor Juan Manuel de Rosas, and he was jailed for a time.

He bred racehorses in subsequent years, and became a member of the board of the Buenos Aires Western Railway.

He married María Josefa Martínez de Hoz, the daughter of prominent landowners, and had one son.

[1] He affiliated himself with Adolfo Alsina's Buenos Aires-centric Autonomist Party, and in 1875, the party's standard-bearer, President Nicolás Avellaneda, appointed Casares Governor of Buenos Aires; the appointment of Casares, a moderate, contributed to an improvement in relations with provincial Caudillos from the hinterland, and fomented national unity.

Amid resurfacing tensions, Casares headed the Autonomist Party committee that nominated General Julio Roca ahead of the 1880 presidential election.