His attendance at the infirmary was so remiss that he more than once fell under the censure of the ‘house visitors,’ and in 1754 he was called upon to resign his office.
He left Bristol and practised for a short time at Oxford, but without much success, and he finally retired to Yorkshire, where he died.
He married, in 1730, Catherine (d. 1782), daughter of Richard Thompson, a wine merchant of York, and by her had a daughter Anne, who married in 1749 Nathaniel Wraxall of Mayse Hill, near Bristol, and by him became the mother of Sir Nathaniel William Wraxall, who wrote the ‘Historical Memoirs of my Own Time.’ Thornhill claims notice as one of the earliest English surgeons to adopt and improve the operation of suprapubic lithotomy.
The records of his work, published by his colleague, John Middleton, M.D., prove that his experience in the operation and his success were greater than any contemporary English surgeon could show.
He was a handsome man, of polished manners, and habitually wore an entire suit of black velvet with an elegant steel-handled rapier.