Third of four children and third son of John of Gaunt and Katherine de Roet. Grandson of King Edward III of England and Phillipa de Hainhault, and Sir Payne de Roet.
Husband of Lady Margaret de Neville, the daughter of Sir Thomas de Neville of Horneby and Joan de Furnivalle. They were the parents of Henry de Beaufort who died very young.
Thomas was legitimitised in 1397 by Parliament, even though he was born before his parent's marriage. He was invested as a Knight, Order of the Garter circa 1400, became the Earl of Dorset on 05 July 1411, Lieutenant of Aquitaine in 1413, the 1st Duke of Exeter on 18 November 1416, Lieutenant of Normandy in 1416, achieved the title of Lord of Lillebonne on 01 July 1418, and the title of Count of Harcourt on 1 July 1418.
Thomas was a military commander during the Hundred Year War and the Chancellor of England 1410 to 1412. He held many posts including the constable of Ludlow, admiral of the fleet of the northern parts, and admiral of the northern and western seas for life. He commanded forces against the northern rebellion in 1405.