[1][2] He joined the navy on 23 February 1780, at the age of 13, becoming a midshipman aboard HMS Royal Oak, under Captain Sir Digby Dent, who would become Mounsey's patron.
[1] The Raisonnable then returned home with despatches, but the ship was so badly damaged by a gale whilst crossing the Atlantic, that she was paid off on her arrival for large-scale repairs.
Dent and Mounsey transferred again, this time to HMS Repulse, initially under Vice-Admiral George Darby, but later being detached to land stores at Gibraltar in April 1781.
[1] Whilst blockading Bastia, he led a dangerous attempt to cut-out a vessel laden with gunpowder, lying the harbour.
On 6 July 1801 he led the boats from Clyde and her escorts to destroy the beached wreck of HMS Jason, coming under heavy fire as they did so.
During his time aboard her he was tasked with carrying despatches, reconnoitering enemy positions, escorting convoys, and conducting anti-smuggling patrols.
Whilst travelling with the convoy on 2 July, a suspicious sail was sighted astern, and Mounsey dropped back to investigate.
[3] In doing so he lost sight of the convoy and in sailing to rejoin them, came across a French frigate on 5 July, which was in the process of capturing an English merchant.
[3] After a chase lasting 18 hours the Bonne Citoyenne caught up with the French ship on the morning of 6 July and brought her to battle.
[3] The enemy ship was discovered to be the Furieuse, which had sailed from the Îles des Saintes on 1 April, carrying sugar and coffee to France.
The captured frigate was later commissioned into the Royal Navy as HMS Furieuse, whilst the Bonne Citoyenne returned to England in September.
Henry Phipps, First Lord of the Admiralty wrote to Mounsey Sir, - I did not fail this day, to lay before his Majesty the particulars of your conduct in the attack and capture of the Furieuse, French frigate, on 6 July.
[3][4] Mounsey and the Furieuse were initially employed in escorting a convoy to the Mediterranean, after which she joined the fleet blockading Toulon under Admiral Edward Pellew.
[4] The capture of the harbour eliminated an infamous corsair haven, and provided an anchorage for Royal Navy ships watching Naples.
[4] For the rest of 1813 Furieuse formed part of Admiral Sir Josias Rowley's squadron, and was present at the capture of Via Reggio, and the unsuccessful assault on Livorno in December.