He was born at Ednam in Roxburghshire, and educated at the University of Edinburgh. It was while a student there that he first published some of his poems, their subject matter mainly being the Jed Valley where he had been brought up. He had been intended for a career in the church, but gave up his divinity course because his sermons were criticised as being too flowery.
In 1725 he went to London, where he met other literary figures including a fellow-Scotsman, David Mallet. He quickly became successful, and won favour with Frederick, Prince of Wales, whom he supported politically. His collected poetry was published as The Seasons in 1730, and he became tutor to the son of Sir Charles Talbot, then Solicitor-General. Afterwards, he became a secretary in chancery and his next major work, Liberty (1734) was dedicated to the Prince of Wales. He also wrote several plays, including The Tragedy of Sophonisba (1734) and collaborated with Mallet on the masque, Alfred, which contained the song, Rule Britannia, and was first performed at Cliveden, the country home of the Prince and Princess of Wales. After Talbot's death, however, Thomson fell out of favour with the prince, and his career ended with The Castle of Indolence, his best-known work, which was published just before his own death.