A Somerset squire, whose family had acquired Mells at the dissolution of the monasteries, Horner sat as a Tory for his county with Sir William Wyndham in Anne’s last Parliament. In 1715 he successfully contested Wells, afterwards narrowly escaping arrest on the discovery of Wyndham’s plans for a western rising.
In 1729 Horner’s wife, who had already inherited Melbury as the co-heir of her brother, succeeded on the death of her sister, the Duchess of Hamilton, to the rest of the Strangways estates. Seven years later she arranged a clandestine marriage between their only surviving child, a 13 year-old daughter, and Stephen Fox, the brother of her paramour, Henry Fox, though her husband strongly objected to the match on the ground not only of the age of the bride but of the politics of the bridegroom. She attempted to appease him by representing that the marriage had taken place without her knowledge, also procuring a written undertaking from Fox not to interfere in county politics.