In 1789 he was appointed captain of his troop, and continued to serve with the hussars, who became so much attached to him, that during the tumults of 1794 he contributed greatly, by his influence, presence and example, to retain under their colours nearly the whole of the regiment, which like the regiment of Royal Germans and the Hussars de Saxe, seemed disposed to desert _en masse_. Thanks to the patriotic zeal displayed by Kilmaine in the cause of his adopted country, the officers of noble family who chose to become emigrants were alone lost to the service; but this proved to him a new source of advancement, and he was soon appointed a _chef d'escadre_, which in the French army is equal to the rank of a general officer, being commander of a division; and about this time he enjoyed the friendship of his countryman, the Comte O'Kelly, who was ambassador of France at Mayence, with an income of 30,000 livres per annum.
As a chef d'escadre Kilmaine served throughout the first campaigns of the Revolution, and under Dumourier and Lafayette commanded a corps of that army which burst into the Netherlands and annexed that territory to republican France.
He fought with remarkable bravery at the great battle of Gemappes, on the 6th November, 1792, and with his hussars repeatedly charged the Austrians, driving them _sabre à la main_ along the road that leads from Mons to Valenciennes; and so pleased was his general, the unfortunate Dumourier, that in the moment of victory he named him colonel; but this nomination was not confirmed by the minister of war. However, he was soon after gratified by a brevet of maréchal de camp, which made him, in rank, second only to a lieutenant-general.
He continued to serve with this army, and to be one of its most active and able officers, during all the sufferings which succeeded the victory at Gemappes. It consisted of forty-eight battalions of infantry, and three thousand two hundred cavalry. In December, by the neglect of the Revolutionary Government, these troops were shirtless, shoeless, starving and in rags; fifteen hundred men deserted; the cavalry of Kilmaine were soon destitute of boots, saddles, carbines, pistols and even sabres; the military chest was empty, and six thousand troop and baggage horses died at Lisle and Tongres, for want of forage. "To such a state," says Dumourier, "was the victorious army of Gemappes reduced after the conquest of Belgia!"