Sir William Trussell Junior was the second son of Sir William Trussell of Kibblestone, Staffordshire and Billesley, Warwickshire. The younger Trussell's biography is difficult to disentangle from that of his father.

With his father, he took up arms for Thomas of Lancaster against King Edward II at Boroughbridge on March 22nd 1322. The two later fled beyond the seas after Lancaster's overthrow, but, like his father,he had probably returned by 1326.

It appears to have been the son who had to fled the country for a second time after King Edward's murder and stayed away while Roger Mortimer remained in power (1327-1330); for the father acted as ambassador and seems to have retained his escheatorship between the failure of Henry of Lancaster's movement of insurrection, at the end of 1328, and the fall of Mortimer in October 1330.

William Junior was, however, back in England in 1329, by which time he had been appointed Constable of the Royal Castle at Odiham in Hampshire. It is probable that it was also this Trussell who was Admiral of the Fleet, west and north of the Thames, in 1339 and 1343. William held the post of Constable of Odiham Castle for the best part of a quarter of a century and entertained King Edward III there several times. In January 1347, he became the custodian of the great Scottish warriors, William De Ramsey and Walter De Halyburton who had been captured by the English at the Battle of Neville's Cross the previous year. Their fellow prisoner, King David Bruce of the Scots was initially sent to the Tower of London, but by early 1355, he too was transferred to Odiham after the collapse of ransom negotiations. The monarch remained under William Trussell's charge for the next three and lived in comfort, if not luxury, within the castle walls. The two appear to have become good friends. Trussell accompanied King David to London to address both the English and Scottish Royal Councils concerning his release and also attended King Edward at Ludgershall (Wilts) to discuss the matter. When David was finally set free, he specifically requested that the Constable of Odiham accompany him to the North. It appears that William was a little reluctant to travel so far, for the English King wrote to him insisting that he not only to go to Scotland, but first he was to journey to London and on pilgrimage to Canterbury. The party left for London on 8th September 1357 - stopping the night at William Trussell's manor at Shottesbrooke in Berkshire on the way. Following their pious detour, the journey to Berwick took just eleven days. 

William was the step-son and chosen heir of King Edward II's favourite, Oliver De Bordeaux, and it was through this man that he inherited his Berkshire estates. These were originally centred around the manor of Foliejon in Winkfield, very close to the Royal Court at Windsor. However, the King insisted he swap these for Eaton Hastings in the north of the county in order that he could extend the Great Deer Park. It was in 1335 that he purchased Shottesbrooke from a London Vintner and it is here that he mostly resided, along with his wife, Isabelle (died pre-1348), and two children, John (who predeceased his father) and Margaret, wife of Sir Fulk Pembridge of Tong Castle (Shropshire), his eventual heiress. In 1337, William founded an ecclesiastical college at Shottesbrooke and built a church there for the attendant warden, five chaplains and two clerks. This survives completely intact to this day and a highly elaborate unmarked double-tomb there is said to be that of William and his first wife; though he did remarry, to Ida, sister and co-heiress of Edward Boteler.