Before being highly successful in politics, Sewell proved to be an extremely adept law student, performed as a violinist, and an orchestral composer, who once was selectively placed in "the lead position of an amateur orchestra" by the first member of the British royal family to live in British North America during any term longer than a visit (1791-1800), Prince Edward, Duke of Kent and Strathearn (at the time known as Prince Edward Augustus).

It is noted that Sewell's "extreme faith" in his attendance and due to his varying roles within his life caused him to be "easily the most powerful official (in Lower Canada and Montreal) under the Governor in the colony.

Sewell was retained by the Governor of Lower Canada at the time (Sir Craig) to "analyze the political ills of the colony."

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.

Some time within 1796-1797, Sewell established an intelligence network within Lower Canada (alongside others like Montreal magistrate and merchant John Richardson) that "would function for more than a decade with relative effectiveness".

[1] Sewell helped introduce the Better Preservation Act of 1797, which allowed the suspension of habeas corpus in cases of suspected treason.

Additionally, Sewell attempted to infiltrate the Roman Catholic Church with centralized employees and leaders loyal to the Executive Council to control their "ignorant" and "superstitious" followers.

Sewell expressed to Colonial Administrator, Lieutenant Governor Sir Robert Shore Milnes that "given the independence of the church and the ignorance and superstition of the population, the influence exerted over the inhabitants by the clergy and the bishop was (both) immense and highly dangerous (in opposing the government's will)".

Finally, Sewell threw in his legal opinion for how they could accomplish this without upset or alarm on behalf of the people recognizing this infiltration as he stated that the "right of nominating the Bishop, the Coadjutor and the Parish priest which (the British government) assumed by the conquest of Canada but has never yet exercised."

Later, in 1805, (Head Bishop) Denaut decided to petition the king for legal recognition of his office in the form of letters patent under conditions to be determined by the crown - Sewell saw that as "a tactical victory."

Therefore, Sewell (along with Jacob Mountain and Sir Milnes) "worked out the details of a scheme for government-financed and -directed elementary schools in the countryside staffed by loyal Canadian teachers who would instruct habitant children in the English language and the blessings of British rule."

Sewell more than once had "stretched the evidence so as to invite acquittal for non-violent property crimes carrying the death sentence, and in some cases, including convictions for murder, he intervened to save a prisoner from the gallows."

However, if Sewell's work was biased towards the Crown, it was still done expertly and with elegance, being notably "remarkable for their clarity of expression, their search for general principle, and the depth of scholarship that underpins them."

"[1] Sewell also led an amateur orchestra and performed violin in a quartet at Quebec City and opened the Theatre Royal there in 1832.

Some time in the 1820s "for many years", Sewell presided over the Quebec branch of the British and Foreign Bible Society, and was a leading member within the Cathedral of the Holy Trinity.

In December 1808, Sewell "assumed the patronage of a literary society formed by (one of his former pupils) Philippe-Joseph Aubert de Gaspe plus other young men of Quebec.

[1] In October 1818, Sewell was appointed to the board of the Royal Institution (of the Advancement of Learning, later McGill University) and headed a meeting of the managers of the Quebec Dispensary.