He began his military career in the service of The Netherlands as a lieutenant in a brigade of Scots Highlanders. He participated in the assault and capture of Bergen-op-Zoom. He then obtained a commission in the 60th or royal American regiment, of which he was for some time an adjutant. He served as a captain in the expedition of Major General James Wolfe in 1759 for the conquest of Canada. He was afterward appointed to the command of the New York independent company, with which he was present at the Battle of Ticonderoga. During the battle he was severely wounded. He was again wounded during the action that preceded the surrender of Fort Niagara. At the end of the Canadian war he returned to England. During the American Revolutionary War he was promoted to the rank of colonel, he and his men were instrumental in the defeat of Benedict Arnold at Quebec. The garrison consisted of 50 fusileers and 350 Highland emigrants, and 700 militia and seamen. Sir Guy Carleton, 1st Baron Dorchester was occupied with arrangements for the general defense of the colony, so the defense of the town was entrusted to Maclean. Some of the faint-hearted and disaffected were now inclined to open the gates to the enemy, but were held in check by Maclean, who guarded the gate with his Highlanders, forbade all communication with the besiegers, and fired upon their flag, an ensign of rebellion, with the result that, after Richard Montgomery was killed, Benedict Arnold abandoned the siege and left the country. Colonel Maclean was subsequently stationed at Fort Niagara, and participated in the Battle of Eutaw Springs with his regiment. He was promoted to brigadier-general after leaving North America.