Lieut.-Colonel Reynell resigned the appointment of Deputy Quartermaster-General in India, and was brought on full pay as Major of the Ninety-sixth regiment on the 5th of May 1808, and on the 22nd of September following was appointed Major in the Seventy-first regiment.
In October 1808, Brevet Lieut.-Colonel Reynell embarked as Military Secretary to Lieut.-General Sir John Cradock, who had been appointed to command the forces in Portugal, and landed in November at Lisbon. He remained in Portugal until April 1809, when Sir John Cradock was superseded in the command of the forces in Portugal by Lieut.-General Sir Arthur Wellesley. Lieut.-Colonel Reynell afterwards accompanied Lieut.-General Sir John Cradock to Cadiz, Seville, and Gibraltar, of which latter place Sir John Cradock was appointed Governor, and Lieut.-Colonel Reynell remained there as Military Secretary until September, when he returned to England.
Lieut.-Colonel Reynell joined the Seventy-first regiment at Brabourne-Lees Barracks in December 1809, immediately after its return from Walcheren. In September 1810 he embarked at Deal with six companies of the Seventy-first regiment for Portugal, landed at Lisbon towards the end of that month, marched soon after to Mafra, and thence to Sobral, where the six companies joined the army under Lieut.-General Viscount Wellington. In October Lieut.-Colonel Reynell had the honor of being particularly mentioned by Viscount Wellington in his despatch, containing an account of the repulse of the attack of the French at Sobral on the 14th of that month. The British army shortly afterwards retired to the lines of Torres Vedras, and Brevet Lieut.-Colonel Reynell was appointed Assistant Adjutant-General to the fourth division under Major-General the Honorable George Lowry Cole.
Early in March 1811, the army of Marshal Massena broke up from its entrenched position at Santarem, and retreated to the northward. Brevet Lieut.-Colonel Reynell entered Santarem with the fourth division the day after Marshal Massena had left it, and continued in the pursuit of the French army to the Mondego. In the affair of Redinha he had a horse killed under him. From Espinhal the fourth division was ordered to retrograde, and recross the Tagus, for the purpose of reinforcing Marshal Sir William Carr Beresford. In 1811 he joined the Marshal at Portalegre, and being the senior British assistant adjutant-general, was directed to join Marshal Beresford’s head-quarters, and proceeded with him to Campo Mayor, from which the enemy retired; was also present at the capture of Olivença, and subsequently accompanied the Marshal to Zafra, between which place and Llerena a smart skirmish occurred with the enemy’s hussars. In May 1811, Lieut.-Colonel Reynell returned to England from Lisbon with despatches from Viscount Wellington.
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 at 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 trans-shipped 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.
In April 1820 Major-General Reynell was suddenly ordered to proceed to Glasgow, having been appointed to the staff of North Britain as a Major-General, in which country he remained until March 1821, when, in consequence of the tranquillity of Scotland, the extra general officer was discontinued. Immediately after he was appointed to the staff of the East Indies, and directed to proceed to Bombay, for which presidency he embarked in September following, and where he arrived in March 1822. After remaining there a month, Major-General Reynell was removed to the staff of the Bengal Presidency, by order of the Marquis of Hastings. In August Major-General Reynell proceeded up the Ganges, and took the command of the Meerut division on the 3rd of December 1822.
The next operation of importance in which Major-General Reynell was engaged was the siege of Bhurtpore. Early in December 1825 a large force had been assembled for this purpose, to the command of which he had been appointed, when, just as the troops were about to move into the Bhurtpore states, General Lord Combermere, the new Commander-in-Chief in India, arrived from England, and Major-General Reynell was then appointed to command the first division of infantry. He commanded that division during the siege, and directed the movements of the column of assault at the north-east angle on the 18th of January 1826, when the place was carried, and the citadel surrendered a few hours after. For this service he was appointed a Knight Commander of the Bath, as well as honored with the thanks of both Houses of Parliament.