The following year, he moved to Quebec City and qualified as a lawyer there. In 1790, he served as interim attorney general for the province. In 1793, Sewell was named solicitor general and inspector of the king's domain and, in 1795, he became attorney general and advocate general in Lower Canada. In 1796, he was appointed judge in the Vice-Admiralty Court at Quebec. On September 24 that same year, he married Henrietta, daughter of chief justice William Smith. He was elected to the Legislative Assembly of Lower Canada for William-Henry (later Sorel) in 1796. In the house he was often called on to draft bills, but with regard to government business he normally played a role secondary to that of leaders of the English party such as John Young and Pierre-Amable de Bonne. He supported the party, except on two controversial issues — the financing of prisons in 1805 and the expulsion of Ezekiel Hart, a Jew — in which his legal opinions obliged him to break rank. He remained in the assembly until 1808.