According to Roland Saint-Clair, writing in the late 19th century, the "best" theory as to the origin of William St. Clair, is that found in the Genealogie of the Sainteclaires of Rosslyn manuscript which was written in 1690 by Father Richard Augustine Hay (1661 – c.

According to Bernard Burke, William St. Clair, was made sheriff of the County of Edinburgh for life in 1271,[1] and this date was also given by historian Will Grant.

[1] Around the same time William St. Clair acquired the Knights Templar lands of Gourton from Walter fitz Stephen de Meliville.

[10] They were later permitted to support Edward I of England in his wars in France, as hostages, but Rotuli Scotiae shows that Sir William de St. Clair was among those who were sent as prisoners to the Tower of London.

On 7 April 1299, Edward I of England issued two years' protection for Amicia, widow of William de St. Clair, dwelling by the king's leave in the county of Edinburgh.