Mercer's six-gun horse artillery troop arrived too late for the Battle of Quatre Bras, but it fought with the cavalry rearguard covering the army's retreat to Waterloo. The troop fought on the extreme right wing of Wellington's army at Waterloo, before being moved into the thick of the fighting nearer the centre of the line. There it beat off repeated charges by French heavy cavalry, disobeying orders to abandon the guns and retire inside nearby infantry squares as the enemy closed. The location of this action is marked by a memorial on the Waterloo battlefield. After the battle, Mercer's troop marched on Paris with the Allied armies, and formed part of the army of occupation.