He was the eldest surviving son of John Selwyn MP of Matson House and his wife Mary, the daughter of General Thomas Farrington. He was educated at Eton College and Hart Hall, Oxford and studied law at the Inner Temple. He succeeded his father in 1751, including the inheritance of Matson House in Gloucester. Selwyn spent 44 years in the House of Commons without being recorded as making a speech. He put his electoral interest, as the person who controlled both seats in Ludgershall and one in Gloucester, at the disposal of the King's ministers, because he was financially dependent on obtaining sinecure offices and a pension, which offset his expenses of bribing the electorate, and his gambling debts. He was MP for Ludgershall in 1747–1754 and for the constituency of Gloucester 1754–1780. After he lost his interest in Gloucester, Selwyn was again member for Ludgershall from 1780 until his death in 1791. He was Mayor of Gloucester for 1758 and 1765. He was also elected for the Scottish constituency of Wigtown Burghs in 1768, when he thought he might be defeated at Gloucester. He was the first Englishman to be elected to Parliament by a constituency in Scotland. He chose to retain the English seat. Selwyn was a friend of Horace Walpole, and a member of the Hellfire Club. He was known for his fascination with the macabre and other forms of sexual eccentricity. He was also a keen observer of public executions. When Selwyn visited a dying Henry Fox, he was refused admission. When Fox learned of this he quipped, "If Mr. Selwyn calls again, show him up. If I am alive, I shall be glad to see him, and if I am dead, I am sure he will be delighted to see me!" He died unmarried in 1791 and was buried in the Selwyn vault at St. Katherine's church, Matson, Gloucester on 6 February 1791. However, the vault has since been filled in and the brass plate from his coffin is now to be found on a wall inside the church. He left his estate to his adopted daughter, Maria Emilia Fagnani, who married Francis Seymour-Conway, 3rd Marquess of Hertford.
George Augustus Selwyn at the age of fifty-one by Hugh Douglas Hamilton, a pastelle drawn in 1770. This drawing is also in the possession of the Earl of Carlisle at Castle Howard.
