Fonblanque was the author of the very extensive notes forming the useful body of the standard legal work, Treatise on Equity nominally ascribed to Henry Ballow.
He is supposed to have written the celebrated letters to the King on the subject of his Royal Highness's exclusion from the army which were generally attributed to Lord Moira.
Made King's Counsel on 28 April 1804, his brilliant early reputation meant he was spoken of as a future Whig Lord Chancellor.
Vanity led Fonblanque into debt in 1807–08, and money problems, his wife left him in 1834, remained with him and his family the rest of his life.
John Anthony Fonblanque died on 4 January 1837 in his 77th year, still confined to the area just outside the debtors' prison, retaining his faculties to the last and "with perfect resignation".