William was the son of Justice Thomas Essex of Walham Green in Middlesex and his wife, Elizabeth daughter and heriess of William Babthorpe of Ellistown in Leicestershire. He was named after his illustrious grandfather, the Treasurer to King Edward IV. When still a child, his father arranged for him to marry Elizabeth, the much sought-after heiress of Thomas Rogers whose vast Berkshire estates included Lambourn, Benham Valence, Odstone in Ashbury and Beckett in Shrivenham. If not actually married in 1487, the two were certainly betrothed by then, when William was only ten and Elizabeth thirteen. Poor Elizabeth's father died just two years later and the now wealthy teenagers settled at Lambourn Place.

William first appears in a position of importance in 1509 at the funeral of King Henry VII, for which he was made an esquire to the body of the new King. However, he somehow disgraced himself soon afterwards and was, unfortunately, thrown in prison for two years. In 1512, he redeemed himself by joining Henry VIII's forces campaigning in France. He became a captain and was knighted at the fall of Tournai in the following September. From then on, he became an important Royal courtier, attending the marriage of Princess Mary and King Louis XII of France, the 'Field of the Cloth of Gold' and the meeting with the Holy Roman Emperor at Gravelines in 1520. It was probably in the early 1520s that Sir William became a close associate of the Lord Chamberlain, William, Lord Sandys of the Vyne at Sherborne St. John, near Basingstoke, when his son, Thomas, married Sandys' daughter, Margaret. In 1528, Sandys was sent to put down a rising of unemployed clothworkers in Westbury and he did so whilst pretending to go hunting on the Essex estates at Lambourn. Four years later, they were trustees together for the Aldermaston estates of Humphrey Forster, another of Sandys' sons-in-law. Sir William also obtained the wardship of Edward Darell of Littlecote Park near Hungerford who was later to marry his grandaughter, Elizabeth.

It was during this period that Sir William was elected as an MP for Berkshire, certainly in 1529 and probably earlier, though the records do not survive. Like Sandys, he was not best pleased with the religious changes which swept the country after Henry VIII's divorce from Catherine of Aragon and, although he did serve at Anne Boleyn's Coronation and accepted the position as her steward in Newbury, he met frequently at the 'Queen's Head' in London with other concerned MPs. Amongst them was Sir George Throckmorton who had openly admitted advising the King not to marry Queen Anne. The King's chief minister, Thomas Cromwell, became suspicious of these diners and many of their names appear on a list he drew up, in 1533, of those opposed to the Bill in Restraint of Appeals, as well as a later one covering those in some way connected with the Treasons Bill. In the Autumn of 1536, Sir William met Sir George in London and borrowed from him a copy of 'Ashe's Manifesto,' the demands of the Northern rebels then taking part in the Pilgrimage of Grace. This was a dangerous move, for distribution of these demands was seen as  tantamount to supporting the rebellion. Sir George later travelled to Lambourn Place to meet with Sir Anthony Hungerford of East Shefford (a relative of Sir William's sister-in-law). On the way, he stopped off at Coley Park, near Reading, to visit the local JP, Thomas Vachell. Thomas was about to leave for London to report the spread of seditious rumours in the area and warned Sir George of the explosive nature of his manifesto. Sir George promptly burnt his copy before spending the night with his brother-in-law, Sir Thomas Englefield, at Englefield House, where there was further serious talk of being more circumspect. At Lambourn, he found Sir William in something of a state, having spent a sleepless night worrying about the growing threat to his liberty. For his servant, Geoffrey Gunter, had surreptitiously copied the manifesto and had begun circulating it  amongst disgruntled priests in Reading. Sir George immediately had him pack his son off to London to hand the culprit over to the  authorities, reassuring him that he would then be in the clear. However, on 18th December 1536, both Sir William and Sir George were imprisoned in the Tower of London. Their lives were in serious jeopardy and Lady Essex must have spent a fretful Christmas awaiting news of her husband's fate. Fortunately, they were both released after just over a month. 

Having been re-instated to Royal favour, Sir William attended the christening of Prince Edward and, eager to prove his loyalty, accepted a post, in 1537, investigating rumours of the King's death being spread in Reading. He was present at the welcoming of the new protestant queen, Anne of Cleves, at Blackheath in 1539 and was later sent to examine a priest who had been reported for having praised the Pope. In 1543, he was one of those who sat in judgment on the Windsor Martyrs who, though protestant, were too extreme for the King's liking. However, Sir William was obviously uncomfortable with the prospect of sentencing them to death and the final verdict was left to his subordinate, the previously mentioned Thomas Vachell. The same year, Sir William further supported the King and his policies by supplying fifty foot soldiers for his futile war in France.

Sir William continued to serve in parliament until his death on 13th August 1548. The lands inherited by his son, Thomas, had changed considerably since the parents' marriage but were still largely in Berkshire - where Sir William had often been High Sheriff - despite exchanging some with the King for former monastic manors. Although, Sir William also at a town-house in the parish of  St. Clement Danes and rented other land in Westminster. He asked to be buried in Lambourn Church under a substantial monument, not unlike that to his son which still exists today. However, no sign of it remains and, in all probability, it was never built.