On entering the King's service Knighton gave up his medical practice, but he did not give up medicine. Artist and fellow Devonian Samuel Prout, incapacitated by headaches, overcame his embarrassment at approaching his former physician and asked for a prescription that Knighton had successfully prescribed for him many years earlier. Harriett Arbuthnot was also grateful for a prescription from Knighton, despite calling him 'a great rogue and a blackguard'. On retirement to the country after George IV's death Knighton gave medical advice to anyone who asked.
Knighton's treatments were conservative. He favoured bleeding, recommending his wife to apply not more than half a dozen leeches to her temples if she felt headachy or bilious. He disapproved of cold-water bathing as a remedy. He enjoyed his food, but preached and practised moderation. When he travelled to Spain with Richard Wellesley he took a portable bed to avoid lice, and warned his party of the dangers of bad water. On occasion, perhaps believing that hope was his only remaining remedy, he gave reassurance that he must have known was ill founded. But it was also said that he was able to gain control over his patients' minds, much to their benefit.
- Aspinall, Arthur, ‘George IV and Sir William Knighton’, English Historical Review, volume 55, No 267 (Jan 1940) pages 57–82 [Canning's daughter]
- Bamford, Francis, and the Duke of Wellington (editors), The Journal of Mrs. Arbuthnot, 1820–1832, 2 volumes (London: Macmillan, 1950)
- Butler, Iris, The Eldest Brother. The Marquess Wellesley, the Duke of Wellington’s Eldest Brother (London: Hodder and Stoughton, 1973)
- Crainz, Franco, An Obstetric Tragedy. The Case of Her Royal Highness The Princess Charlotte Augusta (London: William Heinemann Medical Books, 1977)
- Fletcher, Ernest, Conversations of James Northcote R.A. with James Ward, on Art and Artists. Edited and arranged from the manuscripts and note-books of James Ward (London: Methuen and Co., 1901)
- Grenville, Richard, the Duke of Buckingham and Chandos, Memoirs of the Court of England during The Regency, 1811–1820. From original family documents, volume 1 of 2 (London: Hurst and Blackett, 1856)
- Jennings, Louis J. (editor), The Croker Papers. The Correspondence and Diaries of the late Right Honourable John Wilson Croker, 2nd edition, volume 1 of 3 (London: John Murray, 1885)
- Knighton, Lady [Dorothea], Memoirs of Sir William Knighton, Bart., G.C.H., Keeper of the Privy Purse during the Reign of His Majesty King George the Fourth. Including his correspondence with many distinguished persons, 2 volumes (London: Richard Bentley, 1838)
- Le Marchant, Sir Denis, Memoir of John Charles, Viscount Althorp, Third Earl Spencer (London: Richard Bentley, 1876)
- Lewis, Judith Schneid, In the Family Way. Childbearing in the British Aristocracy, 1760–1860 (New Brunswick, New Jersey: Rutgers University Press, 1986)
- British Library Add 4163 ff217-218 [about cold water bathing]
- Prout correspondence at the National Art Library, Victoria & Albert Museum