He also learned a great deal from Antonio Verrio and Louis Laguerre, two prominent foreign decorative painters then working in England.

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".

[1] 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".

[1] 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".

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.

In 1711, Thornhill was one of the twelve original directors of Sir Godfrey Kneller's academy at Great Queen Street, London.

In November 1724, Thornhill made a second, more successful, attempt to establish a new free academy in his private house at Covent Garden.

Apart from full-size copies, completed in 1731, he made a set at half-scale, plus 162 smaller studies of heads, hands and feet intending to publish them in printed form for the use of art students, but left this work unfinished at his death.

Thornhill's copies of the cartoons were sold by auction by Christopher Cock on 24 and 25 February 1735 at his room in the Great Piazza, Covent Garden.