being in high favour at the time, obtained the commission. He designed for this purpose eight scenes from the life of St. Paul, which he executed in monochrome. These paintings, though in themselves not wanting in grandeur of conception or dignity of design, proved from the outset quite inefficient, owing to the enormous height of the dome and the thickness of the intervening atmosphere. Some of Thornhill's original sketches for this series are in the British Museum, together with other more finished drawings, probably executed by Thornhill for the purpose of a set of engravings which were published soon after. A series of eight finished designs, prepared by the artist to be submitted to Queen Anne, was purchased in 1779 by the dean and chapter of St. Paul's. While Thornhill was painting in the dome his life was saved by the timely presence of mind shown by his assistant, Bently French. Repeated restorations have destroyed anything of interest which remained in Thornhill's work.
Thornhill's paintings in Greenwich Hospital are the most generally familiar among his works. He was engaged on them for about twenty years. Thornhill's services were in great requisition for the decoration of the houses of the nobility and gentry. Blenheim, Easton Neston, Wimpole, Chatsworth, Eastwell, and other well-known mansions contained decorative paintings by him. Comparatively few remain, their destruction being due to neglect and change of fashion rather than to any fault in Thornhill's painting, for his technical method of mural painting possessed great durability and merit. This is especially shown in the fine series of paintings executed by Thornhill for Thomas Foley at Stoke Edith, near Hereford, where he adorned the staircases and saloon with the stories of Cupid and Psyche, and of Niobe, and in one architectural piece added full-length portraits of his patron and himself. At Oxford, where native art at this date was greatly patronised, Thornhill executed paintings at All Souls', Queen's, and New Colleges, but his works have for the most part been destroyed or superseded. His sketch-books, one of which is in the British Museum, show him to have been an industrious and capable artist, with considerable inventive powers, although to suit the conventions of fashion he appears to have kept a kind of register of allegorical and mythological subjects suitable for the various walls or ceilings which he might at any time be called upon to decorate. A sketch-book, with drawings made by Thornhill at Harwich and on the continent, is in the possession of Felix Cobbold, esq., at Ipswich. Thornhill was a capable portrait-painter, and among his sitters were Sir Isaac Newton, Sir Richard Steele, Dr. Bentley, and other famous men.
Thornhill was one of the pioneers of a national school of art. He submitted to the government a scheme for the foundation of a royal academy of painting, to be situated at the upper end of the Mews (near the present National Gallery). Although this scheme obtained the approval of Charles Montagu, earl of Halifax [q. v.], not even that nobleman's influence at the treasury was able to secure its realisation. In 1711 when an academy of painting was opened in Great Queen Street, Lincoln's Inn Fields, with Sir Godfrey Kneller as governor, Thornhill was one of the twelve original directors elected by ballot. A few years later factions arose in the academy, which led to the secession of one group of artists under Thornhill, who started a new academy at a house in James Street, Covent Garden, close to his own house in the Piazza, to which he had removed from his original residence at 75 Dean Street, Soho. Another group of artists, under Cheron and Vanderbank, established a rival academy in St. Martin's Lane. Admission to Thornhill's academy was by ticket, but William Hogarth [q. v.], who attended it, says that it met with little success and was soon closed. In 1724 Thornhill reopened it, but apparently again without success. After Thornhill's death the furniture of this academy was acquired by Hogarth for use in the newly constituted academy in St. Martin's Lane. Thornhill succeeded Highmore as serjeant-painter to the king in March 1719–20, and was knighted in the following April, being the first native artist to receive that honour. Although Thornhill frequently complained of the scale of pay for his paintings, he amassed sufficient wealth to be able to repurchase the old seat of his family at Thornhill in Dorset. He sat from 1722 to 1734 as member of parliament for Melcombe Regis, to the church of which he presented an altar-piece of his own painting, representing ‘The Last Supper.’
Thornhill died at his seat at Thornhill on 13 May 1734. By his wife Judith he had one son, John Thornhill, who succeeded his father as serjeant-painter shortly before his death, but was otherwise of little note; and one daughter, Jane, who was clandestinely married to William Hogarth at Old Paddington church on 23 March 1729. Lady Thornhill survived her husband, and appears to have resided with the Hogarths at
