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!"
Honourable testimony has been given to the unceasing efforts of Kilmaine to preserve order among his soldiers amid these horrors; and with other staff-officers, he frequently endeavoured by private contribution to make out a day's subsistence for their men, who roved about in bands, robbing the villages around their cantonments at Aix-la-Chapelle, and in revenge many were murdered by the peasants when found straggling alone beyond their out—posts.
After the defection and flight of General Dumourier, Kilmaine adhered to the National Convention, and by that body was appointed a general of division; and now he redoubled his energies to restore order in the army, which by the defection of its leader was almost disbanded; thus, in one month after General Dampierre took command, so ably was he seconded by Kilmaine, the discipline was completely established.
He commanded the advance-guard of Dampierre in the new campaign against the allied powers, on the failure of the congress at Antwerp on the 8th of April, 1793; and his leader bears the highest testimony to the gallantry and noble conduct of Kilmaine, in the "murderous affairs of the 1st and 2nd May;" in which, according to the official report, he had two chargers shot under him.
Six days of incessant skirmishing succeeded, during which Kilmaine never had his boots off, nor returned his sabre once to the scabbard; and he displayed the most reckless valour on the 8th of May, in that battle fought by Dampierre to deliver Condé.
The French were routed with great loss; Dampierre was slain; and on Kilmaine as an active cavalry officer devolved the task of covering the retreat of the infuriated and disorderly army, which fell back from Condé-sur-l'Escaut, which is a barrier town, and was then the nominal lordship of the unfortunate Duke d'Enghien.