In November 1775, McGill was a member of the representation for the citizens of Montreal at the drafting of the articles of capitulation for the city to the invading American army. McGill was elected to the Legislative Assembly of Lower Canada for Montreal West in 1792 and appointed to the Executive Council in 1793. He was elected again in 1800 and in Montreal East in 1804. He was the first honorary lieutenant-colonel of the 1st Battalion, Montreal Militia, a unit which later evolved into the Canadian Grenadier Guards, as commemorated by the replicated cairn that stands before the Arts building of McGill University.