Promoted captain on the outbreak of war with Spain in 1739, Mostyn took part in the operations against Cartagena in 1741. In 1745 his failure to engage two French ships off Ushant gave rise to a public outcry and service criticism, not allayed by his acquittal by a court martial ordered at his own request. Two years later he incurred further criticism for failing to attack a weakly escorted French convoy, mistaking the East Indiamen for ships of the line.1Richmond, Navy in the War of 1739-48, ii. 114-15; iii. 78-80. After contesting Scarborough unsuccessfully in 1744, he was returned in 1747 as a government supporter for Weobley on Lord Weymouth’s interest, which he owed to his cousin, Lord Winchilsea. In 1749 he was one of a number of admirals and captains summoned to the Admiralty to discuss an unpopular government proposal to subject half-pay officers to courts martial; the only officer to speak in favour of the proposal, he was rewarded a month later with the comptrollership of the navy.2Augustus Hervey’s Jnl. 81.
Mostyn died 16 Sept. 1757, leaving £60,000 to his nephew, afterwards Sir Roger Mostyn, M.P.3UCNW Baron Hill mss.