[1] He became a professional soldier in various Scottish regiments but was made redundant in 1707 following the Union of England and Scotland.
Douglas was elected as Member of Parliament for Lanark Burghs, also known as Linlithgow Burgs, in 1708 and was returned there unopposed in 1710.
In 1713 he was elected MP for Orkney and Shetland,[2] although this was the first time that the constituency had been contested, usually going to the Morton interest.
[3] He was returned as MP for Lanark Burgs in 1715 when he was unopposed, but in the 1722 general election he was defeated there in a contest.
[4] He surrendered his seat in 1730 when he succeeded his elder brother in the earldom and was elected a Scottish representative peer, which he remained until his death.