Sir James Thornhill was an English painter of historical subjects working in the Italian baroque tradition. He was responsible for some large-scale schemes of murals, including the "Painted Hall" at the Royal Hospital, Greenwich, the paintings on the inside of the dome of St Paul's Cathedral, and works at Chatsworth House and Wimpole Hall.

Thornhill was born in Melcombe Regis, Dorset, the son of Walter Thornhill of Wareham and Mary, eldest daughter of Colonel William Sydenham, governor of Weymouth. In 1689 he was apprenticed to Thomas Highmore (1660–1720), a specialist in non-figurative decorative painting. He also learned a great deal from Antonio Verrio and Louis Laguerre, two prominent foreign decorative painters then working in England. He completed his apprenticeship in 1696 and, on 1 March 1704, became a Freeman of the Painter-Stainers' Company of London.

Thornhill decorated palace interiors with large-scale compositions, with figures commonly shown in idealized and rhetorical postures. In 1707 he was given the commission to decorate the Hall now known as the Painted Hall at the Old Royal Naval College (1707–1727). The scheme of allegorical wall and ceiling decorations of the hall depicts the Protestant succession of English monarchs from William III and Mary II to George I.

On 28 June 1715 Thornhill was awarded the commission to decorate the dome of St Paul's Cathedral by "a whig, low-church dominated committee inspired by a moral Anglican nationalism". The Archbishop of Canterbury, Thomas Tenison, is said to have remarked: "I am no judge of painting, but on two articles I think I may insist: first that the painter employed be a Protestant; and secondly that he be an Englishman". The Weekly Packet said that the decision to award Thornhill the commission would "put to silence all the loud applauses hitherto given to foreign artists". The eight scenes in the dome (1716–19), executed in grisaille, show episodes from the Life of St. Paul.

Thornhill's vast murals in great houses often related to topical events, as seen through the eyes of his mainly Whig patrons. At Chatsworth, during 1707-8 Thornhill painted a number of walls and ceilings, the most notable being the continuous wall and ceiling painting of the Sabine room, then a lobby, but since used as a bedroom. Here he painted The Rape of the Sabine Women, a vast panorama of mounted warriors carrying off the Sabine women to Rome. He chooses to feature strongly Hersilia, who was deified for her loyalty to her Roman husband, Romulus, as against her Sabine family - a deliberate reference to Mary, lauded by the Whigs for supporting her Protestant husband, William, against her Catholic father, James.

At Hanbury Hall, beneath an imposing view of both the Olympian Gods and the story of Achilles which dominates the ceiling of the main staircase, Thornhill added a small portrait of Rev Henry Sacheverell, a Tory propagandist put on trial for sedition by the Whig government in 1710, being cast to the Furies to be burnt. In 1716 Thornhill painted the ceiling of the Great Hall in Blenheim Palace for John Churchill, 1st Duke of Marlborough, newly returned to the country after being prosecuted by the Tory ministry in the last years of Queen Anne. The subject is, inevitably, the Duke's 1704 victory at the Battle of Blenheim, during the War of the Spanish Succession.

His last major commission was to paint the chapel at Wimpole Hall; he started work on the preliminary sketches in 1713 and the work was finished by 1724. The north wall has fictive architecture and four Trompe-l'œil "statues" of the four Doctors of the Church. The east wall above the altar is painted with the Adoration of the Magi.

In 1725 he offered to paint decorations for the ceiling of the New Council Chamber at the Guildhall in the City of London. He gave his services free, although he was rewarded with a valuable gold cup. The chamber was later demolished, though some of the paintings – an Allegory of London, and representations of the Cardinal Virtues, personified as naked children – survive.