Nothing is known for certain about this MP’s background, although his name might suggest that his family originated in Yorkshire. In his youth he may have lived at Horsted in east Sussex, where two brothers, William and Clement Thweyt, were said to be dwelling when they obtained royal pardons in May 1415.
It appears, however, that Thwaites was not actively serving as under sheriff at the time of his election to the Parliament summoned to meet on 21 Jan. 1437. Although he is not recorded as possessing property in Lewes, the borough he represented, he was undoubtedly well known to the inhabitants, for he had earlier acted as an attorney for a number of the townsmen, including Thomas White*, and in 1434 he had rendered an account at the Exchequer on behalf of their bailiff. Furthermore, he had often been employed in the law-courts by the prior of the great Cluniac monastery there.
Fees paid for his legal services enabled Thwaites to accumulate a modest amount of property in the north of Sussex. In 1440 he acquired a messuage and some 128 acres of land in Rusper, a few miles from Horsham,
Thwaites attested the parliamentary election for Sussex on 11 Jan. 1442, when Sir Roger Fiennes (whom he had earlier served as under sheriff) was one of those returned. He continued to be associated with Sir Roger, now treasurer of the Household, in later years, notably as his co-feoffee for the conveyance of property in east Sussex from John Chitecroft* to Adam Levelord*.
Thwaites continued to be linked with the Fiennes family after Sir Roger’s death in 1449, for he served Fiennes’s son and heir Sir Richard (afterwards Lord Dacre), as an attorney, as his deputy in the shrievalty in 1452-3, and as an associate in conveyances of property in Southwark. It may be that he too acquired property in that town, for in 1455 he assisted Thomas Warham in his acquisition of a local inn called The Clement.
A petition filed in Chancery after Thwaites’s death, at an unknown date between 1467 and 1473, reveals something about his first wife, Margaret, one of three daughters of Katherine Blast of Crawley and the mother of his first-born son, who was named William after him. It was stated that following Margaret’s death, when young William was aged about 12, his grandmother Katherine had placed a messuage and some 180 acres of land and 30s. rent in Twineham and Cowfold in the hands of feoffees including the boy’s father to hold in trust until he came of age. On reaching his majority William junior, who had been in ward to Lord Dacre, entered the lands and promptly sold them, but connived with the only surviving feoffee, his cousin Thomas Bradbridge, to prevent the purchaser from having seisin. Bradbridge told a different story. He testified that Katherine had had doubts about her grandson’s character, that the latter had sold the property to raise money to get out of prison, and that the sale was made contrary to the purpose of an entail.