This distinguished officer commenced his military career as an Ensign in the Thirty-eighth regiment, his commission being dated the 30th of September 1793. He joined the regiment in January 1794, at Belfast, and in April proceeded with it to Flanders, where it formed part of the army commanded by His Royal Highness the Duke of York. On arrival at the seat of war, the Thirty-eighth regiment was ordered to join the corps under the Austrian General Count Clèrfait, who commanded the troops in West Flanders, and it was attached to the division under Major-General Hammerstein, together with the Eighth light dragoons and Twelfth foot. Ensign Reynell was present in the action on the heights of Lincelles on the 18th of May, and at the battle of Hoglade on the 13th of June 1794. He afterwards served with the army under the Duke of York, and was in Nimeguen when that town was besieged. On the 3rd of December following, when cantoned between the rivers Rhine and the Waal, he was promoted to the rank of Lieutenant in the Thirty-eighth regiment. Lieutenant Reynell served during the winter campaign of 1795, and the retreat through Westphalia to the Weser, and there embarked for England. He accompanied the Thirty-eighth regiment to the West Indies in May 1796, and was present at the capture of the island of Trinidad in the early part of 1797. On the 22nd of July 1797 he was promoted to a company in the Second West India regiment, and joined that corps at Grenada. Captain Reynell quitted Grenada early in 1798, in consequence of being appointed Assistant Adjutant-General at St. Domingo, where he remained until that island was evacuated by the British in September, when he returned to England. In the beginning of 1799 he revisited St. Domingo, as one of the suite of Brigadier-General the Honorable Thomas Maitland, then employed in framing a commercial treaty with the negro chief Toussaint L’Ouverture, who had risen to the supreme authority at St. Domingo. When it was concluded, Captain Reynell returned to England in July of the same year. On the 8th of August 1799 Captain Reynell was transferred to a company in the Fortieth regiment, with the first battalion of which he embarked for the Helder in that month, and joined the army, which was at first commanded by Lieut.-General Sir Ralph Abercromby, and afterwards by the Duke of York. Captain Reynell was present in the action of the 10th of September; also in the battle of the 19th of September, when he was the only captain of the first battalion of the Fortieth regiment that was not killed or wounded; he was also present in the subsequent battles of the 2nd and 6th of October. Captain Reynell, upon the British army being withdrawn from Holland, re-embarked with the first battalion of the Fortieth regiment, and arrived in England in November 1799.