Douglas was sent at an early age to the court of Louis XIII, where he was served the King as a Page, where he steadily moved through the levels of the Maison du Roi.
At the age of twenty, he was appointed colonel of the Scots Regiment (one of five Scottish units in French service in this period), the first of three brothers to do so.
He was killed in a skirmish on the road between Arras and Douai on 21 October 1645, in an attempt to take the latter city from the Habsburgs.
According to Fraser, Louis XIV had indicated his wish to raise Douglas to the rank of Field Marshal, on the very day that he died, though the appointment was never made.
The Régiment de Douglas returned to British service in 1662, and by 1812 it took its more famous name: The Royal Scots.