This painting is a comparatively rare example, we can be reasonably confident that the sitter was not a Nobleman as the full-dress gown in both Universities for those was gold-laced and not black.
That does not preclude the sitter from noble birth, since those entitled to enter the University as a Nobleman were sometimes entered as Gentleman or Fellow Commoners to reduce the fees, as the social privileges were not dissimilar.
There was no particular distinction of dress from College to College in Oxford. In Cambridge there was (and is), so Fellow-Commoners differed in what they wore and quite often there was some gold or silver lace on the gown. Fellow-Commoners of Downing had and still have black-tufted gowns, but this is too early for that College.
Born in Plymouth and apprenticed to his father, a poor watchmaker, in his spare time James drew and painted. In 1769 he left his father and set up as a portrait painter. Four years later he went to London and was admitted as a pupil into the studio and house of Sir Joshua Reynolds. At the same time he attended the Royal Academy Schools. In 1775 he left Reynolds, and about two years later, having made some money by portrait painting back in Devon, he went to study in Italy. On his return to England, three years later, he revisited his native county and then settled in London, where John Opie and Henry Fuseli were his rivals. He was elected associate of the Academy in 1786 and a full academician the following spring. Northcote’s works number about two thousand, and he made a fortune of £40,000. He was elected to the Royal Academy in 1787.
Northcote also sought fame as an author, and his first essays were contributions to the Artist, edited by Prince Hoare.