In July 1811, Brevet Lieut.-Colonel Reynell embarked as military secretary to Lieut.-General Sir John Cradock, K.B., who had been appointed governor and commander of the forces at the Cape of Good Hope, where he arrived by the end of September. On the 4th of June 1813, he received the brevet rank of colonel; and on the 5th of August 1813, he was promoted lieut.-colonel of the SEVENTY-FIRST regiment, in succession to Lieut.-Colonel the Honorable Henry Cadogan, who was mortally wounded in the battle of Vittoria; in February following, being desirous of joining the corps, Colonel Reynell resigned his staff situation at the Cape, and proceeded to England, where he arrived in May 1814. In July of that year he was appointed adjutant-general to the force then preparing for service in America under Lieut.-General Lord Hill; but, other operations being then in view, that appointment was cancelled. Colonel Reynell took the command of the first battalion of the SEVENTY-FIRST regiment at Limerick in December 1814, and embarked with it from Cork in January of the following year, as part of an expedition for North America; but peace having been concluded with the United States, and contrary winds having prevented the sailing of the vessels, the destination of the battalion was changed. In March Colonel Reynell received orders to proceed with his battalion to the Downs, where, in the middle of April, it was transhipped into small vessels, and sent immediately to Ostend, to join the army forming in Flanders, in consequence of Napoleon Bonaparte having returned from Elba to France. In the memorable battle of Waterloo, fought on the 18th of June 1815, Colonel Reynell commanded the first battalion of the SEVENTY-FIRST regiment, and was wounded in the foot on that occasion. He afterwards succeeded to the command of Major-General Adam’s brigade, consisting of the first battalions of the fifty-second and SEVENTY-FIRST, with six companies of the second, and two companies of the third battalion of the ninety-fifth regiment, in consequence of that officer being wounded. Colonel Reynell commanded the light brigade in the several operations that took place on the route to Paris, and entered that capital at the head of the brigade on the 7th of July 1815, and encamped with it in the _Champs Elysées_, being the only British troops quartered within the barriers. In this year he was appointed a Companion of the Order of the Bath, and received the Cross of a Knight of the Austrian Military Order of Maria Theresa, also a Cross of the fourth class of the Russian Military Order of St. George. Colonel Reynell remained with the “_Army of Occupation_” in France until October 1818, when, after a grand review of the united British, Danish, and Russian contingents at Valenciennes, the SEVENTY-FIRST marched to Calais, and embarked for England. Colonel Reynell continued in command of the regiment until the 12th of August 1819, the date of his promotion to the rank of major-general.