[1] He was a cousin of Rear-Admiral Thomas Francis Charles Mainwaring; and was the representative of an ancient Staffordshire family, settled for many centuries at Whitmore Hall, near Newcastle-under-Lyne.

[1][a] He was also on board HMS Majestic at the battle of the Nile, after which, in consequence of Captain Westcott having been killed, he was, in October 1798, removed into the frigate Thalia, commanded by Lord Henry Paulet, under whom he was further employed in the Defence, 74 guns, on different European stations, until within a short period of his being made Lieutenant, on 7 December 1801, he was appointed lieutenant of the Harpy sloop, Captains Charles Worsley Boys and Edmund Heywood.

[4][5] The following is the copy of an official letter written by Sir Peter Parker, who was once Mainwaring's messmate in the Leda: HMS Menelaus, off Villa Francha, 1 March 1812.Sir,– I feel great pleasure in acquainting you of the capture of a beautiful French brig, on her first voyage, named the St. Joseph, from Genoa, laden with naval stores for the arsenal at Toulon.

This service was performed last night, by Lieutenant Rowland Mainwaring, first of the Menelaus, in a masterly manner, near the Bay of Frejus, where the St. Joseph was moored within pistol-shot of a battery flanked by another; also by musketry from the shore.

[7]In July 1812 the Menelaus was cruising on the coast of Italy, and Sir Peter Parker reported the capture of the French xebec La Paix, mounting two long 6-pounders, with a complement of thirty men, "under circumstances peculiarly honourable to Lieutenant Mainwaring, who boarded and brought her out from within pistol-shot of the towers of Terracina, under a galling fire.

"[8] During the night of 2 September 1812, the French letter of marque St. Esprit, pierced for twelve guns, but with only two six-pounders mounted, was cut out from the river Mignone, near Civita Vecchia, under a heavy fire from the batteries.

[10] He was subsequently placed by that officer in the Kite sloop, and sent to the Archipelago, where he destroyed a French privateer, rescued a valuable merchantman which she had captured, and obtained from the Bey of Salonica a promise, that in future no vessels of the same description should be equipped in his harbours.

[10] He afterwards commanded the Paulina sloop, in which he obtained restitution of two merchant vessels, taken by an American privateer and carried to Tripoli, where he remained watching the enemy until the final cessation of hostilities, thereby preventing her from giving any further annoyance to the British trade in the Mediterranean.

[10] Rowland Mainwaring married, first, in January 1811, Sophia Henrietta, only child of Major William Duff, 20th Regiment, and daughter-in-law to Captain George Tobin, Royal Navy, CB.