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.
Mostyn died 16 Sept. 1757, leaving £60,000 to his nephew, afterwards Sir Roger Mostyn, M.P.
