The Rookwood family had become prominent in Suffolk by the mid 14th century. William’s grandfather, Robert, was knight of the shire in 1348, and his father, John, twice represented the county in Parliament, in 1360 and again in 1368. The substantial family lands were concentrated in the area of Stanningfield in the west of the county, but for the most part these passed at John’s death in about 1386 to William’s half-brother, another John. William himself was the child of his father’s second marriage and, following the death of his mother in 1392, he inherited the Rookwood properties in Acton, Long Melford and Groton near the Stour, which had been settled specifically on her and her issue. By the time of his own death he had also acquired three manors in Acton, ‘Ingham’ in Brockley, and holdings at Hartest, Whepstead and Lawshall, which, altogether, provided him with a comfortable income in excess of £25 a year. He acted as executor of his mother’s will and always remained close to her wealthy kinsmen the Swinburnes, whose principal seat was across the border in Essex.
Rookwood may well have been a lawyer by profession, for throughout his career he was kept busy as a feoffee of estates in Suffolk and Essex, as an executor, and as a mainpernor for local landowners, tasks often entrusted to those with a knowledge of the law. In many of these dealings he was associated with his half-brother, John, whose marriage to one of Sir William Burgate’s daughters brought the Rookwoods other important connexions among the local gentry. John served as alnager of Suffolk in 1394-5 and escheator of Norfolk and Suffolk a few years later, but William’s involvement in the administration of his home county did not begin until after the accession of Henry IV. Before then he had stood surety at the Exchequer in 1392 on behalf of the local lawyer Robert Hethe, and three years later he had acted as a feoffee of the property of Richard Withermarsh. In 1397 both he and his half-brother were involved in financial transactions for his uncle, the important military captain, Sir Thomas Swinburne, from whom they received a ‘gift’ of all his goods and chattels, doubtless to safeguard them from confiscation. A lifelong connexion of William’s was that formed with the Essex lawyer, Richard Baynard of Messing, whose mother may have come from the Rookwood family, for many years earlier William’s father had been a trustee of Baynard’s parents’ estates, and Baynard could refer to our MP as cognatum meum. In 1398 Rookwood witnessed an enfeoffment made by Walter, Lord Fitzwalter, by whom his colleague was retained as legal counsel. In December 1399 when he set up a trust for his own property, the feoffees he chose included, besides his uncle, Swinburne, and his friend, Baynard, such noteworthy figures as Sir Robert Corbet (his wife’s kinsman), Sir Andrew Butler, Sir Richard Waldegrave (son of the Speaker of that name) and Robert Tey. Clearly, he was a well known figure among the gentry of the shires of Suffolk and Essex.
In the will Sir Thomas Swinburne made in August 1412, his nephew, Rookwood, was given the exacting task of administration with only one colleague to help him. This involved prolonged dealings with the royal Exchequer, for Swinburne had served as mayor of Bordeaux and, at the time of his death, as captain of the great fortress of Fronsac; his accounts had to be settled and negotiations undertaken for the recovery of large sums of money owed him by the Crown. In January 1415 Rookwood and his fellow executor obtained a formal grant of Swinburne’s estates in the province of Perigord, which they were then enabled to sell.
In 1417, after a few months employment as a j.p. in Suffolk, Rookwood was returned to Parliament for the shire, apparently for the first time. He subsequently sat in three more Parliaments, in the last of which his kinsman and friend, Richard Baynard, then representing Essex, was elected Speaker. He continued to provide services to associates and neighbours in their private transactions: in 1418 he helped Edmund Wynter to purchase the reversion of the manor of Hengrave and then, two years later, to sell it to Thomas Hethe; in June 1421 his mother’s half-brother, William Swinburne, named him as an executor; and in 1422 he assisted John Spencer’s widow, Katherine Burgate (his late brother’s sister-in-law), to make settlements of manors in Norfolk.
Rookwood died on 26 Apr. 1422, leaving as heir to his property his son John, then aged 26, who, however, only survived eight years more.