Short answers, pulled from the story.
James Thornhill arrived in the world on the 25th of July 1675 or perhaps 1676 within the Dorset town of Melcombe Regis. His father Walter worked as a merchant while his mother Mary came from a family of governors and colonels.
In 1707 James Thornhill received a massive commission to decorate the hall now known as the Painted Hall at the Old Royal Naval College. The project would consume twenty years of his life until its completion in 1727 with allegorical wall and ceiling decorations depicting the Protestant succession of English monarchs.
On the 28th of June 1715 James Thornhill won the competition to paint the dome of St Paul's Cathedral because a committee dominated by low-church Whigs selected him over foreign artists who had previously held such prestige. The Archbishop of Canterbury Thomas Tenison insisted on two conditions that the painter must be Protestant and he must be an Englishman.
On the 2nd of May 1720 the King knighted James Thornhill making him the first native artist to receive such honor. He succeeded his former master Highmore in the latter role that same year and entered politics when he returned unopposed as Member of Parliament for Melcombe Regis at the 1722 British general election.
In 1711 James Thornhill joined twelve original directors of Sir Godfrey Kneller's academy at Great Queen Street in London and later established his own private drawing school at Covent Garden though it soon closed. A second attempt in November 1724 proved more successful as he created a new free academy in his house where William Hogarth seems to have been a member from the beginning.