He was educated at a Roman Catholic school in Douai in France, as was then common with some of the Catholic highland Scottish aristocracy (Catholic educational institutions being illegal in Scotland).[1]
He is best known for his command of the infantry defending the Hougoumont farmhouse at the Battle of Waterloo in 1815, where he fought with great distinction, including leading the vital closing of the gates, with his sergeant, James Graham. The farmhouse had around 1,000 defending it against a French force of 2,500. MacDonell was wounded slightly in the battle.[3][4]
The Duke of Wellington awarded him £1,000 for his part in the battle calling him "the bravest man in the British Army". MacDonell insisted on splitting this money with his sergeant, James Graham.[1][5]
In 1830, he was promoted to major general and posted to Ireland, where in 1837 he had to abate the troubles in Armagh.[1]