The regiment moved to Boston in June 1775 for service in the American Revolutionary War. It fought at the Battle of Long Island in August 1776, the Battle of White Plains in October 1776 and the Battle of Fort Washington in November 1776. It went on to take part in the Philadelphia campaign, seeing action at the Battle of Brandywine in September 1777, the Battle of Paoli later that month and the Battle of Germantown in October 1777. It was following the British attack on the Americans at Brandywine, where the light company of the regiment took no prisoners and the Americans demanded vengeance, that the regiment decided to insert identifying red feathers in their shako helmets to prevent anyone else suffering on their account. It then sailed for the West Indies in summer 1778 and took part in the British victory at the Battle of St. Lucia in December 1778 before returning to Ireland in 1780. In 1782, the regiment received a county distinction becoming the 49th (the Hertfordshire) Regiment of Foot.