He was named colonel in the militia during the Upper Canada Rebellion and commanded the left flank of the loyalist forces under Colonel James FitzGibbon at the Battle of Montgomery's Tavern. McLean initially opposed the Union of Upper and Lower Canada out of fear that Upper Canadians would be dominated by French Canadians. He saw responsible government as a danger to the British connection and to the ordered freedom and the recognition of class and property of the British tradition, but he quickly adjusted to the new reality.