He served as a captain in the expedition of Major General James Wolfe in 1759 for the conquest of Canada.

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.

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.

[3][5] Sir Allan entertained Samuel Johnson and James Boswell on Inch Kenneth in October 1773.