In 1443 he and his father were appointed as stewards to the Duchy of Lancaster's estates in Northamptonshire, Buckinghamshire, Bedfordshire and Huntingdonshire, and by 1446 Thomas was serving as an esquire for Henry VI, being made an usher of the king's chamber in 1455.
Shortly after leaving home on 23 September they were attacked by a group of men involved in a property dispute with his father; William Tresham was killed, and Thomas was injured.
Tresham stayed in favour throughout the disturbances of 1456, and was again made High Sheriff of Cambridgeshire and Huntingdonshire between 1457 and 1458, and for Sussex and Surrey between 1458 and 1459.
As a result, he took part in the plots of John de Vere, 13th Earl of Oxford, and was imprisoned in the Tower of London from 1468 until Henry VI regained the throne in 1470.
A daughter, Isabella, was born in 1460 and married Sir Henry de Vere of Addington, thus establishing a long line of descendants.