On the outbreak of the War of 1812 he joined the 3rd Regiment of York Militia as a Subaltern and was seriously wounded at the Battle of Queenston Heights.

Because of an infection caused by the late removal of a bullet he was not fit to fight when the Americans attacked York in April, 1813.

In 1820, he was elected to the Legislative Assembly of Upper Canada for Stormont; he held that seat until 1834, shared for the previous four years with his old school friend Philip VanKoughnet.

Appointed to the Legislative Council in 1836, the following year he was named to the Court of King's Bench, his brother Alexander McLean taking his Stormont seat.

The Upper Canada Law Journal wrote, Upon the bench was dignified and courteous; unsuspicious and utterly devoid of anything mean or petty in his own character, his conduct to others was always what he expected from them.