Being a minor aged 15 at his father's death, he became a ward of Nicholas Seagrave until 1293, when he recovered livery of his estates. By 1295, Ferrers was abroad on royal service, and acting as Edward I's agent at the Duke of Brabant's court in Hainault. Although he was short of money at the time (having had to mortgage the Newbottle manor for £200), this did not prevent him taking part in the King's military campaign. Other royal service included on the Scottish Marches under both King Edwards in their various campaigns there. In 1296, under the King's instruction, the Keeper of Scotland, John de Warenne restored Ferrers to his Scottish estates that the King still held. He fought at the Battle of Falkirk on 22 July 1298 and at the Siege of Caerlaverock two years later. Following the siege, the Keeper of Galloway also by order of King Edward, restored to Ferrers those estates in that region that the King had still held. In 1301 William Ferrers was signatory to the (eventually unsent) Barons' Letter of 1301 to Pope Boniface VIII, in which Ferrers and 95 other English barons and five English earls repudiated the Pope's claim to overlordship of the Kingdom of Scotland.and defended the aggressive policy of King Edward I.