Mariana Alcoforado was born in Beja, daughter of landed proprietor of Alentejo Francisco da Cunha Alcoforado, born at Cortiços, Macedo de Cavaleiros, and first wife Leonor Mendes. She had three brothers: Baltasar Vaz Alcoforado, Miguel da Cunha Alcoforado and Francisco da Cunha Alcoforado, and two sisters: Anna Maria da Cunha Alcoforado, wife of Rui de Mello Lobo Freire, and Maria Peregrina Alcoforado. Beja was the chief garrison town of the province and the principal theatre of the twenty-eight years' war with Spain that followed the Portuguese Revolution of 1640. Mariana's widowed father, occupied with administrative and military commissions, placed her in the wealthy convent of the Conception for security and education. He later remarried and had two more daughters, Maria da Conceição Alcoforado and Catarina Alcoforado, and was also made a Knight of the Order of Christ on December 15, 1647.