During 1688 Harley acted as his father's agent in promoting support for William, Prince of Orange and the Protestant cause against the policies of James II. When William landed in England on 5 November, Sir Edward Harley and his son immediately raised a troop of horse in support of the cause of William III, and took possession of the city of Worcester on his behalf. Harley was sent to report to William, meeting him at Henley. Harley obtained a commission as a major of militia foot in Herefordshire, which he held for several years.