Thomas George Southwell, 3rd Baron and 1st Viscount Southwell, was the eldest son of Thomas Southwell, 2nd Baron Southwell of Castle Matress and his wife Mary Coke. Educated at Christ Church, Oxford and Lincoln's Inn, he joined the Coldstream Guards in 1738. In 1747, he became MP for Enniscorthy and later Limerick County, before he succeeded his father as Baron in 1766. He was also an active freemason, serving as Grandmaster of the Grand Lodge of Ireland, and was elevated to the title Viscount Southwell in 1776. On 18 June 1741, he married Margaret Hamilton, daughter of Arthur Cecil Hamilton of Castle Hamilton, Killeshandra, Co. Cavan. He died in 1780.