Mathews joined the navy in 1690 and saw service on a number of ships, including during the Nine Years' War and the War of the Spanish Succession. He interspersed periods spent commanding ships with time at home at the family estate in Llandaff. He distinguished himself with service with Sir George Byng at the Battle of Cape Passaro in 1718, and went on to command squadrons in the Mediterranean and Indian Ocean, before largely retiring from naval service. He returned to active service in 1741, following Britain's entry to the War of the Austrian Succession, and took command of the fleet in the Mediterranean. The usual difficulties of performing delicate diplomatic duties were further exacerbated by the fact that he was on bad terms with his second in command, Richard Lestock, on whom he relied to manage the fleet. The pivotal moment of his naval career came in 1744, when he attempted to intercept a Franco-Spanish fleet at the Battle of Toulon. The action was fought in confused circumstances, with poor communications and the breakdown of the chain of command. Despite possessing the superior force, Mathews was unable to secure a decisive result, and the enemy were able to escape with the loss of one ship, while Mathews's fleet lost one and had several others badly damaged.