Alexander Cavalié Mercer (28 March 1783 – 9 November 1868) was a British artillery officer. Although he rose to the rank of general, his fame is as commander of G Troop Royal Horse Artillery in the thick of the fighting at the Battle of Waterloo, and as author of Journal of the Waterloo Campaign.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.Mercer's Journal is an important source for historians of the Waterloo campaign, as well as a detailed description of the landscape and people of Belgium and France in the early 19th century. It is one of the few accounts of the period written by an artillery officer.Mercer remained in the peacetime army, twice serving in Canada. He was a painter of some merit, and a number of his watercolours of Canadian landscapes were purchased by the National Gallery of Canada in the 1980s.