During the Jacobite rising of 1715, Douglas took the Hanoverian side, and led the volunteer horse at the Battle of Sheriffmuir. In maturity, Douglas grew eccentric, and perhaps insane, slew Captain John Kerr (the natural son of his uncle, Lord Mark Kerr) at Douglas Castle, while the latter was guesting with him there, and was forced to flee for some time to Holland. His residence of Douglas Castle was plundered by Highlanders during the Rising of 1745.