Sewell helped introduce the Better Preservation Act of 1797 which allowed the suspension of habeas corpus in cases of suspected treason. In 1797, he prosecuted David McLane for treason; McLane was executed. He prepared legislation which led to the establishment of the Royal Institution for the Advancement of Learning (later McGill College) in 1801. In 1808, he was named Chief Justice for Lower Canada and became a member of the Executive Council of Lower Canada. Later in 1808, he was appointed to the Legislative Council and was named speaker in 1809. Sewell supported a union of Upper and Lower Canada; however, in 1822, he opposed a legislative union because of the strong opposition to union in the province. In 1809, he published rules of practice for the Quebec Court of King's Bench and the Court of Appeals; James Monk published similar rules at Montreal, Lower Canada.