He was articled to Thomas Fulljames (1808–1874) in Gloucester and acted as clerk of works for the latter's Edwards College, South Cerney (Glos) in 1838–39. He was elected to the Royal Institute of British Architects in 1837, on the nomination of George Basevi, Edward Blore and William Railton, and became a Fellow of the Institute in 1856, proposed by Benjamin Ferrey, Giles Gilbert Scott, and Francis Penrose. He twice served on the Council of the Institute (in 1858–60 and 1870–72). He was Surveyor to the Middle Temple in London from 1851 until 1885, and practised from Lambe Buildings in the Temple for much of his career. From about 1885 onwards, when he seems to have semi-retired, St. Aubyn worked in partnership with Henry John Wadling (d 1918), who entered his office as a pupil in 1858 and remained as his assistant and managing clerk. St Aubyn died on 7 May 1895 at Chy-an-Eglos, Marazion (Cornwall), and is buried on St Michael's Mount. H J Wadling succeeded to his practice, and continued to trade as "St Aubyn & Wadling".