William was heir of Otto Grandson (d. 1328), head of the English branch of a noble Savoyard family whose principal residence was Grandson Castle near Lake Neuchâtel.
Younger brother of Otes de Grandison, he was in the service of Edmund, Earl of Lancaster, who, on his behalf sent a letter to the King (when William's lands were seized, he being an alien), pointing out the undesireability of such seizures. On 4 Nov 1288, he had letters of protection when remaining in Wales in order to fortify the castle of Carnarvon. He was excepted from military service in Gascony in 1304, in which year he appears as governor of Jersey and Guernsey for his brother Otes. He was summoned to Parliament from 6 Feb 1298/99 to Oct 1325, whereby he is held to have become Lord Grandison. He was again in Gascony with the Earl of Lancaster before 1 Jan 1295/96, when is lands were restored to him. He was present at the siege of Carlaverock in Jul 1300, and was summoned to the coronation of Edward II on 18 Jan 1307/08. On 26 Nov 1300, it was ordered that the lands of Sir John Tregoz should be divided between William and his wife and the other coheirs. In 1318, the Prior of the Hospital of St. John of Jerusalem made complaint that William and his sons, Piers and Otes, with others, had broken into his houses and robbed and assaulted, to which William responded that there had been theft of his goods there. When he did not attend a muster for military service in 1322, his lands were seized, but as his reason for in attendance had been severe illness, he was excused and given license to remain at home, provided that he send at least 6 men-at-arms for the expedition. In Jun of 1327, he had letters of protection for going to Ireland, and on 20 Sep 1329 had respite of homage until the following Easter, as the King had learned that he was so infirm and aged that he was unable to come. However, he was summoned in Jul 1332, to be with the King at Michaelmas and take passage for the expedition to Ireland. He survived his wife, Sibyl, younger daughter and coheir of Sir John Tregoz, she dying Oct 1334, he following her in June 1335. (Complete Peerage Vol. 6 p. 60-62)