By the outbreak of the War of the Austrian Succession he was a captain, and served with several of the leading naval officers of the day in operations off the British coast, and in the Caribbean.

He entered politics in 1747, representing the constituency of Weobley until his death, and was involved in the administration of the navy as Comptroller, and as a Lord of the Admiralty.

With Deptford Mostyn went out to the West Indies with Sir Chaloner Ogle's fleet, and later served under Vice-Admiral Edward Vernon, during the attempts to capture Cartagena in March and April 1741.

He was present with Norris off Dungeness on 24 February 1744, when the British intercepted a French fleet under the Comte de Rocquefeuil which was carrying troops for a planned invasion of Britain.

After waiting another day and night in the hope that Dreadnought would catch up, Mostyn abandoned the chase, fearing that he was too close to the French shore, and that he would be overwhelmed by the combined firepower of his opponents if he attacked alone.

[3][4] After his return to port, Mostyn justified his actions, claiming that the choppy, squally weather caused his ship to heel over so far as to render his lower gunports inoperable, while the French vessels sat higher in the water and could have used theirs.

[3][4] The Admiralty accepted the explanation, and the matter might have passed, had Mostyn not written to the Navy Board to request his masts be replaced with smaller ones.

[1][5] Mostyn's supporters quickly published a response, A Vindication of the Conduct of Captain M-N, attributing blame solely to Hampton Court's tendency to roll in heavy swell.

[6] Despite this response, public opinion remained against Mostyn and nearly a year after the verdict he was jeered out of Portsmouth Dockyard by workmen and sailors calling out 'All's well!

[2] He was with a squadron when they fell in with a convoy being escorted by Comte Dubois de la Motte on 20 June 1747, and took 48 merchants as prizes.

[1] He was appointed Comptroller of the Navy on 22 March 1749, having to manage the repair of old ships and the construction of new ones during the peace, when the dockyard workforces were being reduced.

[8] Mostyn returned to active service with the outbreak of the French and Indian War, receiving a promotion to rear-admiral on 4 February 1755.