Mostyn was court martialled at his own request in 1745, after allegations that he had failed to engage two French vessels off Ushant, and was acquitted. He sat for Weobley on the interest of Lord Weymouth, but his main political connexion seems to have been with his cousin, Lord Winchilsea. He supported Administration.
When Edward Boscawen sailed for America in April 1755 in command of a squadron, he appointed Mostyn his second-in-command, believing him to have been unfairly criticized.
