The sitter was the eldest son of Sir John St. Aubyn, 4th Bt. (1726-1772) and Elizabeth, daughter of William Wingfield of Durham. He succeeded to the baronetcy and to the family estates at Clowance, near Crowan, Cornwall, and Stoke Damerel, Devon, on 12 October 1777. He was elected Sheriff of Cornwall in 1781 and on 6 February 1784 he entered parliament as M.P. for Truro. In spite of his extensive interests in Cornwall, where he was Grandmaster of the Freemasons between 1785 and 1839, St. Aubyn chose to live in London or at his seat in Essex, Shortgrove Hall. He conducted long-term relationships with two Cornish women, Martha Nicholls (d. 1829) and later Juliana Vinicombe (1769-1856), of Marazion, whom he eventually married on 1 July 1822 at St Andrew's, Holborn. Of his fifteen illegitimate children, he and Juliana had six sons and two daughters before their marriage; the other seven are thought to have been by Martha. The painter J.B. Lane explained to the artist and diarist, Joseph Farington that St. Aubyn's unorthodox lifestyle was due to the influence of a profligate clergyman who had led him at an early age 'into scenes of vice with women and familiarised him to this kind of intercourse' (The Diary of Joseph Farington, XI, January 1811-June 1812, K. Cave, ed., New Haven and London, 1983, p. 3903).
A lover of science and the arts, St. Aubyn was a keen amateur mineralogist. In 1799, he bought two important collections of minerals; firstly, that of William Babbington, who had obtained his collection from John Stuart, 3rd Earl of Bute; and secondly, the fossil and mineral collection of Richard Greene of Lichfield. St. Aubyn was an early patron and friend of John Opie and was a pallbearer at the artist's funeral in 1807. He sat to Opie for three portraits, one of which is now in the Plymouth City Museum and Art Gallery, where St. Aubyn's important mineral collection can be found.