The Percys came from Wiltshire, but William seems to have inherited none of their holdings there. It was through his mother, Elizabeth Hartridge, that he came into possession of the manors of Woodmancote, Southwick and Morley as well as moieties of Truleigh, in Sussex, Wambrook in Dorset, and Weston in Berkshire, his title resting on an entail which, made in 1354, provided that after the death of his mother and her then husband, John Farneburgh, these properties should pass to him and his wife, Mary Filoll. The rest of the Hartridge inheritance went for the most part to his half-sister. Percy’s wife came from an old Dorset family; and her father was to serve as duchy of Lancaster steward in the county in the 1370s and 1380s.
For more than 20 years from 1377, Percy was a prominent figure in the administration of Sussex as a royal commissioner and j.p. In October 1377, while representing the shire in Parliament on the first of 12 occasions, he stood surety at the Exchequer for the lessees of property in Somerset, and in December he acted similarly for Sir Nicholas Tamworth’s widow, who had been granted the farm of certain alien priory estates. Meanwhile, during the parliamentary session, he had been appointed sheriff of Surrey and Sussex. The task was not always easy: in the following spring he made complaint that the mayor and bailiffs of Chichester had assaulted his under sheriff and other subordinates when attempting to hold judicial sessions in the city. Percy had already formed a close attachment to the Sussex baron, Robert, Lord Poynings. They were associated together in 1378 and 1379 as mainpernors in the Exchequer for the farmer of the Sele priory estates, and in 1382 Sir William assisted Poynings to purchase a number of manors. Such was their friendship that when Poynings made his will in July 1386, while preparing to depart for Spain (whence he was never to return), he appointed Percy as his sole executor, authorizing him to retain certain of his lands for as long as 20 years in order to pay his debts and provide for the marriages of his younger children.
An even more important connexion had been formed with Richard, earl of Arundel, whose service Percy entered before 1380, the year in which he witnessed a deed whereby Arundel became a feoffee of the widespread estates of William, Lord Latimer. During the Peasants’ Revolt of 1381 he served under the earl in restoring order in Sussex—a task which he continued as sheriff in the following year, and it was Earl Richard who, in October 1382, petitioned the King for a pardon for his retainer, so relieving him of payment of a fine incurred on account of the escape of prisoners from Guildford castle gaol. Only four months later Sir William was placed on a commission of oyer and terminer to bring to trial insurgents accused of having broken into the earl’s castle at Lewes, burned manorial account rolls and evidences, and consumed casks of wine. He witnessed other deeds relating to the Latimer estates in 1383 and 1384, having in the meantime joined his lord at Petworth as witness to a settlement made by the earl of Northumberland. He was present, early in 1385, when Earl Richard acquired by exchange the London mansion known as ‘Pulteneys Inn’. On the assembly of the fleet to put to sea under Arundel’s command as admiral of England in March 1387, Percy was mustered in the company of the late Lord Poynings’s cousin, Thomas, Lord St. John of Basing. At home again three months later, he acted as mainpernor in Chancery for Sir Richard Cergeaux, husband of the earl’s niece, Philippa. Naturally enough, when his lord took up arms in December that year, and in conjunction with the duke of Gloucester seized power from the King’s favourites, Percy was at his side.
After an absence from the Commons of nearly four years, Percy was elected to the Parliament summoned to Cambridge in September 1388, while Arundel and his fellow Lords Appellant were still in control of the government; but it would appear that, on this occasion, the earl considered him to be of more use at home in Sussex than in the Lower House, for on 14 Aug. the sheriff was ordered to hold a fresh election to find a replacement, thus freeing Sir William to take charge of the array of the men of the shire to resist French incursions along the south coast. He went on to represent Sussex in six of the next seven Parliaments. In the will he made in 1393, Earl Richard left this trusted retainer a silver cup with a cover, and before his trial and execution for treason four years later he named him among the trustees of various of his landed estates, including the reversion of Kenninghall (Norfolk), the castle and lordships of Chirk and Chirksland (Denbighshire) and his London residence. Sir William was therefore party to transactions, completed in 1396 and 1397, whereby Philippa Cergeaux formally relinquished to her uncle and his feoffees all title to the estates of the earldom of Arundel.
Aged about 70, Percy made his will on 19 Mar. 1407, requesting burial in Woodmancote church, where a window on the north side was to be glazed at his expense. Robert, 4th Lord Poynings (the son of his one-time friend), was named as co-executor with the widow, and his young son, William (probably Percy’s godson), was left ten marks. The will was proved at Woodmancote on 2 July, by the rector of Poynings acting on the instructions of the registrar of the court of Canterbury.