Born in Altyre, Moray, Scotland, he died in Buchan and is buried in Deer Abbey.

He helped oversee the construction of St Mungo's Cathedral in Glasgow and after his death, Marjory continued his work there.

During his time as Warden of Moray, Comyn's success may have been a reason for his marriage to Marjory (a.k.a.

Her father, Fergus, Earl of Buchan (died c.1214), had no male heirs and so in marrying his daughter to William he ensured a suitable line for his titles before his death.

[4] For the historian Alan Young, William's life, and particularly his marriage to the Countess of Buchan, marks the beginning of the "Comyn century".