Enjoying the patronage of Captain Hon. Samuel Barrington, Simonton saw early service aboard the Achilles 60 with that officer, from which ship he was commissioned lieutenant on 27 June 1759 and appointed to the Duke 90, Captain Samuel Graves. During the latter part of 1767 he was aboard the Tamar 16 at Plymouth, which vessel had of late been commanded by Captain Henry l’Anson. He later rejoined Captain Barrington, serving aboard the Portsmouth guardship Albion 74 in the spring of 1771.
During the summer of 1775 he was appointed to the storeship Maria in the Downs, being sent out to Virginia from Spithead later that year with ammunition and arms, and with orders to place himself under the command of Captain Henry Bellew of the Liverpool 28. The colonial governor of Virginia, the Earl of Dunmore, appointed him to the Dunmore 22 in December, and he retained her until August 1776 whilst continuing as an agent of transports. He was also appointed agent of transports going out to Jamaica in the same year, but in November was diverted to the Prince of Wales 74, which was being recommissioned in England by his patron, Captain Barrington.
By 1778 he was the senior lieutenant of what was now Rear-Admiral Hon. Samuel Barrington’s flagship Prince of Wales 74, Captain Benjamin Hill. He was promoted commander of the sloop Alderney 10 at Yarmouth on 5 May as a beneficiary of their Majesties Fleet review, and was posted captain on 9 July.
In September 1778 Simonton was appointed to recommission the Superb 74 at Portsmouth as the flag-captain to Rear-Admiral Sir Edward Hughes, leaving England for the East Indies in March 1779. In 1780 he removed to the frigate Sartine 32 in succession to the late Captain Alexander M’Coy, but she was lost on 26 November after hitting rocks during an attack on Hyder Ali’s ships at Calicut, Simonton having previously complained of being ordered to operate too close inshore.
After returning home he joined the Panther 60 in the Downs at the end of June 1782 in place of the sickly Captain Thomas Piercy, seeing duty briefly as the flag-captain of Vice-Admiral Francis William Drake on that station before sailing to St. Helens in early July. He was employed in the latter part of the Channel Fleet campaign of April-August with a flying squadron of four sail of the line and three frigates under the orders of Captain Samuel Reeve of the Crown 64, and on 23 July he re-took the cutter Pygmy in the Bay of Biscay, sending her into Plymouth. After a brief spell cruising off Ireland the Panther entered Portsmouth on 24 August, and at the end of the month went out of harbour to Spithead where she joined the Grand Fleet, which was preparing to sail for the relief of Gibraltar. Raising a commodore’s pennant, Simonton was placed in charge of the convoy of store ships, and the Panther led these vessels into the waters of the Rock on 18 October. In the subsequent action with the Spanish off Cape Spartel his command suffered several casualties. In November she arrived at Cork with Vice-Admiral Mark Milbanke’s squadron, and in the same month set off for Plymouth where she was waiting on Christmas Day to sail out with a convoy.
For a short while in the spring of 1783 the Panther flew the flag of Vice-Admiral Milbanke, who was second in command at Plymouth, but within a matter of weeks of peace being declared her, by now fractious, crew was paid off.
With his frequent applications for further service falling on deaf ears, Simonton became a superannuated admiral living at Greenwich, and he died in the early part of 1797.
He was regarded as an excellent officer.