He was the son of Amicia de Roskelyn and Sir William Sinclair, Baron of Roslin.
The following year, on 24 February 1310, William was one of twelve Scottish bishops to swear fealty to King Robert the Brus.
However, king Edward II of England had his own candidate in mind, John de Leck.
The pope then declared William's election canonical, and sent him to Cardinal Berengar Fredol, bishop of Tusculum, in order to be consecrated.
On 3 February 1313 king Edward II issued a safe-conduct to William, clearly indicating that the bishop was planning to arrive in England on his way back to Scotland, however Edward demanded cooperation in political matters as a condition.