A soldier of fortune, Barclay fought for Gustavus Adolphus, King of Sweden during the Thirty Years' War, and with the Earl of Middleton during the Wars of the Three Kingdoms.

In 1630, he rose to the rank of a major under Gustavus Adolphus, King of Sweden, during the Thirty Years War.

[1] He returned to Scotland in 1636 to serve in the covenanting army, becoming colonel of a cavalry regiment under General John Middleton.

[2] In 1648, he purchased the lands and barony of Ury in Kincardineshire (modern-day Aberdeenshire) from William Keith, 7th Earl of Marischal.

Barclay served at the court of Charles II and he became an influential figure at Whitehall and Edinburgh.