The origins of William are uncertain, the first of the name of Douglas to appear on historic record. He appears as witness to a charter of Jocelin, Bishop of Glasgow in 1174 in favour of the monks of Kelso Abbey, at which time he was in possession of the Lands of Douglas.
David Hume of Godscroft in his history refers to the progenitor of the House of Douglas, Sholto. Gleaned from the works of Buchanan and Boece, Godscroft's narrative explains that during the reign of a King Solvathius, Sholto Douglas was instrumental in putting down an uprising by a usurper Donald Bain in 767AD, and as reward was granted the lands that would after be called Douglas.
Both Balfour Paul and Maxwell agree that this origin tale is mythic, but do contest that William of Douglas was active at the time of the real rebellion of the Meic Uilleim, under their chief Domnall mac Uilleim. The earlier historians may have confused the mythic Donald Bain with Domnall Ban mac Domnaill, the penultimate Meic Uilleim chief.
This may be corroborated by the facts that the lands of Douglas marched with those of the leader of King William I of Scotland's retaliatory forces, Lochlann, Lord of Galloway. William may well have been a vassal of the Lord of Galloway. Furthermore, all of William's sons with the exception of the eldest were to hold privileged ecclesiastic positions within the former Meic Uilleim territories in Moray.
William of Douglas may have married a sister of Freskin of Kerdal, a Flemish laird from Moray. He had issue:
- Archibald I, Lord of Douglas
- Bricius de Douglas, Bishop of Moray
- Alexander de Douglas, a canon of Spynie, vicar capitular of Elgin
- Henry de Douglas, a canon of Spynie
- Hugh de Douglas, a canon of Spynie, Archdeacon of Moray
- Freskin de Douglas, parson of Douglas, later Dean of Moray
- Margaret de Douglas, married Hervey de Keith, Marischal of Scotland
|Offspring of William de Douglas, Lord of Douglas and Unknown de Kerdal (-c1214)|
|Archibald Douglas (bef1198-c1238)||1198||1238||Margaret Crawford (-c1238)|
|Bricius de Douglas (-1222)|
|Alexander de Douglas|
|Henry de Douglas|
|Hugh de Douglas|
|Freskin de Douglas|
|Margaret de Douglas|