In November 1764 Holburne ordered a secret investigation into an apparent plot to set fire to Portsmouth Dockyard. He was promoted to full admiral in 1766. As his port commission progressed, and encouraged by his friend and patron Lord Argyll, he contested the seat of Stirling Burghs at the 1761 General Election, beginning a career in politics. He was returned as MP for Stirling and in 1768 he represented Plymouth, holding the seat until his death in 1771. He was appointed to the Board of Admiralty as Senior Naval Lord under the North Ministry in February 1770, holding office until he was made Governor of Greenwich Hospital, by way of retirement, in February 1771. He lived at Laurel Lodge in Twickenham and died just a few months later, in July 1771.