William appears as early as 1232, witnessing a charter as the son of Ferquhard, Earl of Ross.
He played a pioneering role in the Scottish reconquest of the Hebrides, which had been under Norwegian control.
Hákonar saga Hákonarsonar tells us that in Norway: "In the previous summer [i.e. that of 1262], letters came east from the Hebrides ... and they brought forward much about the dispeace that the Earl of Ross ... and other Scots, had made in the Hebrides, when they went out to Skye, and burned towns and churches, and slew very many men and women ...
1 Uilleam's attacks on Norwegian possessions earned him the ire of King Haakon, who planned an expedition against him.
William died in May 1274 at "Earles Allane", likely the site of modern Allanfearn or Allangrange.