Mordaunt was promoted to commander in April 1703. Through the influence of Lord Wharton, he was elected as Whig member for Malmesbury at the 1705 English general election. He was not active in Parliament, due to his frequent absences at sea but was able to support the Court with regard to the 'place clause' in the regency bill on 18 February1706. In 1706 he was promoted to captain and was given command of , a 70-gun third-rate ship of the line. At the time Britain was at war with France. He was commanding a small squadron of frigates to convoy his father from Spain to Genoa when off the coast of Italy on 19 March 1707 "Resolution" encountered six French vessels, each ranging between 58 and 80 guns. There followed an extended engagement, during which "Resolution" was badly damaged by French cannon fire. Mordaunt ordered the crew to run the ship aground rather than risk her capture by the French; this was achieved and she was subsequently set on fire to prevent her being refloated. The French maintained a continual gunfire on the burning wreck, and Mordaunt was struck in the thigh by a cannonball as he left the vessel. His father reached his destination and returned to England, and was able to obtain a pass for his son to travel home through France. Mordaunt survived the wound but was left unable to walk, and was forced to retire from the Navy.