Lord Lewis followed the example of his father by actively supporting the Jacobite cause. In October 1745 he abandoned his career in the Royal Navy and swore allegiance to Prince Charles Edward Stuart. He was appointed lord lieutenant of Aberdeen and Banffshire, with orders to raise another Jacobite army in the north. When the main army headed south to England in November, Lord Lewis remained in Scotland where he recruited, with difficulty, a regiment and defeated government troops at Inverurie on 23 December 1745. After the Battle of Culloden, Lord Lewis escaped to France. He became increasingly mentally unstable and died in exile in Montreuil, not yet thirty.