Reilly became regimental lieutenant-colonel in 1868, and next year was the guest of Lord Mayo in India, whence he wrote some descriptive letters to "The Times" newspaper. He spoke French fluently, and at the end of October 1870, while the siege of Paris was going on, he was sent out as extra military attaché to the British embassy at Tours. He at once joined the headquarters of the French army of the Loire, and became the channel for distributing British contributions in aid of the wounded. He was present at the Battle of Beaune-la-Rolande, and the subsequent battles in front of Orléans. The hurried evacuation of Orléans by the French in the night of 4 December took place without his knowledge. He was arrested there next morning by the Prussians, and sent to England by way of Saarbrücken and Belgium. He wished to rejoin the British embassy, then at Bordeaux, but the British government decided that he should not. In recognition of his services the French government raised him to the grade of officer of the Legion of Honour on 20 March 1872, and commander on 4 November 1878.