Kelly was born in Holsworthy, Devon on 3 February 1785 and baptised on 1 September 1790. He was the son of Benedictus Marwood Kelly lawyer and private banker, and Mary Coham. He entered the Royal Navy on 19 October 1798 as an able seaman aboard, serving under Captain Philip Wodehouse. He moved with Wodehouse to the 28-gun and then to the 80-gun in November 1799, under the command of his uncle, Captain William Hancock Kelly. Benedictus spent the next six years aboard her, and in her assisted at the capture of Admiral Jean-Baptiste Perrée's squadron of three frigates and two brigs on 19 June 1799. He attended the expedition of 1800 and 1801 to Ferrol and Egypt, and was wounded in a boat attack on the French defences at Portoferraio on the island of Elba. He spent some time on the books of the 100-gun HMS Royal William, the flagship of Admiral George Montagu and the 74-gun under Captain Mark Robinson. Kelly returned to serve under his uncle in October 1804, now in command of the 98-gun. He remained aboard Temeraire after William Kelly was superseded by Captain Eliab Harvey, and on 12 January 1805 was appointed a sub-lieutenant aboard a schooner. He was commissioned as lieutenant on 31 January 1806 and appointed to the 50-gun. Adamant was ordered to escort a convoy of East Indiamen as far as the Cape of Good Hope and on 6 May 1806 he assisted in the capture of the 30-gun Spanish frigateReparadora. In August 1807 he moved to the 32-gun and served under a succession of commanders, Frederick Warren, William Ward, and Samuel Hood Inglefield. Under Inglefield Kelly was present with the squadron under Charles Dashwood in an attack on the town of Samaná in San Domingo on 11 November 1808. The town was captured and the 5-gun privateers Guerrière and Exchange were also taken. Kelly was then given command of the boats of Daedalus and the frigate and sent to chase down and capture the officers and men of the privateers, who had escaped upriver. Kelly was successful in this endeavour, capturing them all after four days of tracking and a fierce skirmish.
Promotion
From Daedalus Kelly moved to become first lieutenant of the 64-gun in March 1810. Polyphemus was the flagship of Vice-Admiral Bartholomew Rowley, who died during his posting. Kelly was sent home with the despatches aboard. He captained the 18-gun sloop during the Invasion of Java between August and September 1811, and was promoted to commander on 28 November 1811. He then spent a period on half-pay without active employment, despite petitioning the Admiralty for a posting during the War of 1812 and Lord Exmouth'sexpedition to Algiers. Kelly was finally given a seagoing commander with an appointment to the 22-gun on 22 September 1818. He served off the coast of Africa until February 1822. For his good service here he was promoted to captain, post-dated to 19 July 1821.