Born at Newport, Fife on 26 October 1869, eldest of the four sons and a daughter of John Prain of Dundee and Sara Elizabeth Leay, his wife. His father was afterwards agent of the Pacific Steam Navigation Co at Panama and at Valparaiso, Chile. Prain was educated at Cheltenham College, 1882-88, and at St Thomas's Hospital, where he served as house surgeon and clinical assistant in the throat department. He was then resident medical officer at the Evelina Hospital for Sick Children, Southwark, and took the Fellowship in 1898. Going out to Chile, Prain qualified there in 1900 and practised first at Concepcion and after 1902 at Valparaiso, where he bought the British Naval Hospital from George Frederick Cooper, MRCS 1859, who was retiring to England. The hospital was destroyed the same year by the disastrous earthquake of 19 August 1906. Prain never married. He continued to practise at Casilla 1213, Valparaiso, where he died on 4 October 1943, a little before his seventy-fourth birthday.