In 1770, Simcoe entered the British Army as an ensign in the 35th Regiment of Foot, and his unit was dispatched to the Thirteen Colonies. Later, he saw action in the American Revolutionary Warduring the siege of Boston. After the siege, in July 1776, he was promoted captain in the 40th Regiment of Foot. He saw action with the grenadier company of the 40th Foot in the New York and New Jersey campaign and the Philadelphia campaign. Simcoe commanded the 40th's Grenadiers at the Battle of Brandywine on 11 September 1777, where he was wounded. Legend has it that Simcoe ordered his men at Brandywine not to fire upon three fleeing rebels, among whom was George Washington.

In 1777, Simcoe sought to form a Loyalist regiment of free blacks from Boston but instead was offered the command of the Queen's Rangersformed on Staten Island on 15 October 1777. It was a well-trained light infantry unit comprising 11 companies of 30 men, 1 grenadier, and 1 hussar, and the rest light infantry. The Queen's Rangers saw extensive action during the Philadelphia campaign, including a successful surprise attack (planned and executed by Simcoe) at the Battle of Crooked Billet. In 1778, Simcoe led an attack on Judge William Hancock's houseduring a foraging expedition opposed by Patriot militia. Hancock was also killed, although he was not with the Americans. The attack took place at night and with bayonets. On 28 June of that year, Simcoe and his Queen's Rangers took part in the Battle of Monmouth, in and near Freehold, New Jersey.

On 26 October 1779, Simcoe and 80 men launched an attack on central New Jersey from southern Staten Island known as Simcoe's Raid, from what is known today as the Conference House, resulting in the burning of Patriot supplies inside a Dutch Reformed Church in Finderne, including hay and grain; the release of Loyalist prisoners from the Somerset County Courthouse; and Simcoe's capture by Armand Tuffin de La Rouërie. Simcoe was released at the end of 1779 and rejoined his unit in Virginia. He participated in the Raid on Richmond with Benedict Arnold in January 1781 and was involved in a skirmish near Williamsburg and was at the siege of Yorktown. He was invalided back to England in December of that year as a lieutenant-colonel, having been promoted in March 1782.