Tench joined His Majesty's Marine Forces, Plymouth division, as a Second Lieutenant on 25 January 1776, aged 17.
He was promoted to 1st Lieutenant on 25 January 1778 at the age of 19 years and 3 months.
He fought against the American forces in their War of Independence, during which he was captured when HMS Mermaid was driven ashore on the Maryland coast at Assateague Island near the then extant Sinepuxent Inlet on the morning of 8 July 1778 by the French under Comte d'Estaing.
Tench was in command of the Marine unit on board HMS Mermaid.
He and the other officers were transported to Philadelphia, imprisoned and exchanged in October, 1778.
Tench was promoted to Captain-Lieutenant in September 1782 and went on half-pay in May 1786.
The retirement did not last long, as in October of that year the Admiralty called for volunteers for a three-year tour of Botany Bay.
Tench's offer to re-enter the corps was accepted in December 1786, and he sailed on the transport ship Charlotte in May 1787.