Lieutenant-Colonel in the British Army. He was killed on 13/Aug/1756, during the French and Indian war in North America, by canon fire, when defending Fort Oswego (of which he was in command) against a French force under General Montcalm. d.s.p.
The French and Indian War
During the French and Indian War, French Commander, General Montcalm, attacked the British in August, 1756 with a force of 3,000 men who included three regiments of regulars, several companies of Canadian militia and a band of Indians.
He first captured Fort Ontario, then began the assault on Fort Oswego. Montcalm swept the fort with cannon fire, killing the British Commander, Colonel James Mercer, in the bombardment. British forces were forced to surrender on August 15, 1756. According to some sources, a large number of them were then slaughtered by the Indians whom the French were unable to keep under control.
Montcalm gave much of the British supplies to his Indian allies and destroyed the fort. He returned to Quebec in triumph with 1,700 prisoners. His actions made a very strong impression on the Indian allies of the British and caused the Onieda and the Senecca tribes to switch to the French side.
(Based on various sources, notably an article at http://en.wikipedia.org)