Callwell was born in London, the only son of Henry Callwell, of Lismoyne, Ballycastle, County Antrim, by his wife, Maud Martin, of Ross, Connemara. He was educated by a German governess, and then at Haileybury, before entering the Royal Military Academy, Woolwich, in 1876. He was commissioned as a lieutenant in January 1878, joining a battery of the 3rd Brigade, Royal Field Artillery, then stationed in India, and serving in the closing stages of the Second Anglo-Afghan War. In January 1881 his battery was transferred to Natal, arriving just in time to take part in the final operations of the ill-fated expedition against the Transvaal Boers. Shortly afterwards Callwell returned to Woolwich; then in late 1884 he passed the entrance examination to the Staff College, where he was a student from February 1885 into 1886. He was promoted to captain on 17 March 1886.