Born on 13 September 1734, Thomas Thynne succeeded his father as 3rd Viscount Weymouth in January 1751 and was Lord Lieutenant of Ireland for a short time during 1765, although he never visited that country.
Before the close of 1768 he was transferred from the northern to the southern department, but he resigned in December 1770 in the midst of the dispute with Spain over the possession of the Falkland Islands.