Son of Manuel Mendes da Fonseca (1785-1859) and Rosa Maria Paulina da Fonseca (1802-1873). His father was also a soldier and belonged to the Conservative Party. He had two sisters and seven brothers and three of them died in the Paraguay War: Afonso Aurélio da Fonseca (the youngest), ensign of the 34º Voluntários da Pátria Battalion; captain Hipólito Mendes da Fonseca, dead in the Battle of Curupaity; and major Eduardo Emiliano da Fonseca, dead in the Battle of Itororó.