On the 10th of March 1805 Major Reynell received the brevet rank of Lieutenant-Colonel upon being appointed Deputy Quartermaster-General to the King’s troops in the East Indies. In July following he was appointed Aide-de-camp to the Marquis Cornwallis, Governor-General of India, and accompanied his Lordship from Madras to Bengal, with whom he remained until his Lordship’s decease at Ghazepore in October 1805. Lieut.-Colonel Reynell returned to Madras immediately afterwards, and was appointed Military Secretary to Lieut.-General Sir John Cradock, the Commander-in-Chief at that presidency. He officiated during several months of the year 1806 as Deputy Adjutant-General in India, in which country he remained until October 1807, when he returned with Lieut.-General Sir John Cradock to Europe, and arrived in England in April 1808. 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 He remained in Portugal until April 1809, when Sir John Cradock was superseded in the command of the forces in Portugal by Lieut.-General Sir Arthur Wellesley.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.