He was commissioned lieutenant on 24 April 1755 and promoted commander of the bomb Mortar 8 on 15 January 1761, cruising in the Bay of Biscay and then going out to the Mediterranean in December 1762 with news of the ending of hostilities. She was paid off in June 1763. He later had the Pomona 18 from 1767-9, initially off the north of Ireland and later on the south coast of England.
Symons was posted captain on 28 January 1771 and in March he set sail in the Northumberland 70 as flag captain to Rear-Admiral Sir Robert Harland, the commander-in-chief in the East Indies. Returning home in 1774, he joined the Cerberus 28 in August 1775, going out to North America and creating a great deal of havoc in attacking Falmouth, Maine. His frigate patrolled to the north of Long Island Sound in the early part of 1776, taking a number of rebel vessels off Block Island in April, and she participated on 8 December at the occupation of Rhode Island. On 13 August 1777 a prize being towed by the Cerberus was subject to a combustible attack by one of David Bushnell’s devices near New London, Connecticut, three men being killed outright. She later rescued the crew of the armed schooner Admiral Parker, Lieutenant Robert Deans, which had ran aground off Stonington on 22 September and was set on fire. The Cerberus was herself abandoned and burned at Rhode Island on 7 August 1778 to prevent capture by the French.
In August 1779 Captain Symons held the temporary command of the Defence 74 for Captain James Cranston, participating in the Channel fleet retreat, and in October 1779 he recommissioned the Brune 32, sailing for the Leeward Islands in February 1780. A few months later he was appointed to succeed Captain Samuel Uvedale aboard the Ajax 74 after that officer had returned home following the Battle of Martinique in April, being employed in the Leeward Islands campaign of May-July, and remaining at St. Lucia when the fleet headed north. The Ajax was driven out to sea during the Great Hurricanes of October, and she took eight days to return under her foremast and the stump of a mainmast. He served with the fleet following the occupation of St. Eustatius on 3 February 1781 and fought at the Battle of Fort Royal on 29 April 1781 off Martinique before leaving her in the next month and departing for Europe in command of the Gibraltar 80 in August with Admiral Rodney a passenger.
He was flag captain of Rodney’s flagship Formidable 98 at the Battle of the Saintes on 12 April 1782, although Captain Lord Cranstoun had joined the ship that very morning in order to replace him. Following the battle he removed briefly to the Resolution 74 in place of the fatally wounded Captain Lord Robert Manners, and in June exchanged with Captain Sir James Wallace of the Warrior 74, sailing to New York with Admiral Hugh Pigot in command of the fleet and returning to the Leeward Islands before going home and paying her off in 1783.
Symons did not serve thereafter due to the severity of his gout but retired to Bury St. Edmunds. He became a rear-admiral on 12 April 1794, a vice-admiral on 1 June 1795 and died on 16 December 1799 of apoplexy.
Lord Rodney was little enamoured of Symons, complaining that he was slow to refit the Formidable at Plymouth prior to going out to the West Indies in the winter of 1781. In turn Symons was happy to confide in Rodney’s deputy, Rear-Admiral Sir Samuel Hood, with whom the commander in chief was also on bad terms, that Rodney was always complaining of his health. He was criticised for his ill-treatment of rebel prisoners when serving in the Cerberus off North America.