One of the leading military commanders of his day, Rempston dedicated his career, as his father had done before him, to the service of the house of Lancaster. With the exception of the ancestral estates at Rempston, almost all the property which descended to him on the death of Sir Thomas the elder in 1406 had been purchased out of the fruits of royal patronage, and it is thus hardly surprising that he kept up the same loyal attachment. The survival of his mother, the redoubtable Margaret Rempston, until 1454 deprived him of part of his inheritance in the manors of Bingham and Clipstone in Nottinghamshire and Hopewell in Derbyshire, as well as of other sizeable holdings in the two counties, but he could still rely on a comfortable landed income. It is unlikely that he came of age much before the accession of Henry V, who bestowed a knighthood upon him on the eve of his coronation. That he stood well with the new King may be inferred from his return to the first Parliament of the reign, which met a few weeks later at Westminster. In July 1413 he was appointed by the Crown to take securities for good behaviour from Sir John Burton II and Sir Ralph (the future Lord) Cromwell, who were then being sued for assault by Thomas Annesley. Since Rempston was actually helping Annesley to prosecute his appeal, he no doubt discharged his duties with relish. Indeed, it is interesting to note that in July 1416 he assumed the wardship of the estates and person of Annesley’s next heir, who was still a minor. Rempston had, meanwhile, obtained his first taste of warfare overseas, fighting with a personal retinue of eight men-at-arms and 24 archers in King Henry’s first invasion of France. He came back to England with the royal army and again took his seat in the Commons in March 1416, but for part of the year he evidently performed garrison duty at Harfleur, as some of his men were accused of desertion and thrown into prison. The autumn was spent on his Nottinghamshire estates, helping Sir John Assheton II to arbitrate in a serious property dispute (which involved Sir William Meryng, and had probably been referred to them on the recommendation of Parliament), although the opportunity for further military conquests abroad led him to take up arms again in the following year, this time for a longer period.
Sir Thomas left England with the royal army in July 1417, returned briefly at the beginning of 1418, perhaps on official business, and spent the next two years campaigning with the King across Normandy. He took part in the triumphant entry of the English into Paris in December 1420, having by then been rewarded with the captaincy of the enemy castles of Bellencombre and Meulan. While abroad he twice agreed to act as a trustee for Sir Robert Plumpton, his kinsman and comrade-in-arms, who died in battle not long afterwards. A trusted member of the royal bodyguard, Rempston accompanied King Henry when he sailed to England in 1421 for the coronation of his queen, Katherine de Valois. Like his royal master, however, he was anxious to return to the theatre of war, and when the fleet embarked again for France in May 1421 he and his retinue of 40 soldiers were on board.
By this date Rempston was the owner of impressive holdings on both sides of the Channel. In France he had been rewarded with land in Caen, the Côtentin, Alençon, Maine and Gacé, which, together with the other profits of war, made him a wealthy man. His assets at home had, furthermore, been greatly augmented by an extremely lucrative marriage to Alice, the only child of Thomas Bekering and his wife, Isabel, who was herself heir to a third of the Lowdham estates. On her father’s death in 1425 Alice gained possession of major properties in Walton and Brimington in Derbyshire, Tunford, Laxton, Bilsthorpe and Lowdham in Nottinghamshire Honington, Winterton and Marton in Lincolnshire, Catworth in Huntingdonshire and Farnborough and Avon Dassett in Warwickshire.
Rempston’s imprisonment lasted for at least seven years and left him heavily burdened with debts, such as a loan of £180 which his friend, Sir Henry Pierrepont, helped him to raise from a London draper. He returned to England in, or just before, July 1436, while some of his ransom was still outstanding, and began to resume the life and activities of a country gentleman. His duties as sheriff and constable of Flint, which he had discharged for many years through a deputy, evidently claimed some of his attention, for he stood bail on various occasions on behalf of local men. He was also much in demand as a witness and trustee, most notably on the part of Sir Robert Plumpton’s son and heir, Sir William, who conveyed to him all his extensive estates in Nottinghamshire, Derbyshire and Staffordshire.
Rempston’s last years appear comparatively uneventful in comparison with the dramatic successes and reversals of his time overseas. From 1449 onwards he served regularly as a royal commissioner and j.p., and in 1450 Henry VI made him one of the temporary custodians of the Calais march. In other respects, however, his life was now generally free from incident, and although he acted briefly as warder of the duke of Exeter during his imprisonment at Pontefract in 1454, he managed to avoid the political upheavals which occurred during this period. Indeed, he was at last able to achieve a measure of financial stability, since the death of his wife’s aunt, Margaret, the widow of Sir John Lowdham, in May 1451, brought him a third share in her dower properties in Nottinghamshire; and that of his mother, three years later, finally released the manor of Arnold and other holdings settled upon her at the beginning of the century by her husband.