António Cabreira was born at 4:30 p.m. on 30 October 1868, in the family home in Tavira, on the corner of Rua Borda de Água de Aguiar and Rua da Alegria (renamed in 1917, Rua Jacques Pessoa and Rua Dr. António Cabreira, respectively). He was the son of General Tomás António da Guarda Cabreira (1822–1886) and his wife D. Francisca Emília Pereira da Silva from Ponta Delgada; on his father's side, he was the grandson of (1792–1834), a field marshal in the Miguelist army during the Portuguese Civil War, who had been granted the titles of Count of Lagos and Viscount of Vale da Mata by the Miguelist faction. António Cabreira was the younger brother of Colonel (1865–1918), a Republican politician who taught Chemistry at the and who briefly occupied the office of Minister of Finance in 1914.