Jonathan Sewell (born Jonathan Sewall; June 6, 1766 – November 11, 1839) was a lawyer, judge, defensive spymaster and political figure in Lower Canada. Sewell utilized the idea of substantive law (shaping how people should act through distilling punishment) over procedural law (outright punishing the guilty for what was committed) as much as possible when it came to delegating punishment for criminal cases specifically; Sewell saw the certainty of punishment over the seriousness of punishment as enough to alter the intentions of non-violent or non-hardened criminals. In civil suits, Sewell "likely did more than anyone to professionalize the administration of civil justice (in Lower Canada and Montreal) prior to the codification of civil laws in 1866."

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).

Sewell had attempted to influence the French-Canadian population in Montreal and Quebec (at the time Montreal and the colony of Lower Canada) in the early 1800s through both a failed attack on the legitimacy of the Roman Catholic Church in the colony, and through control of the educational system the colonial government was responsible with delegating. The latter could be argued as successful as the institution created under the "mind control to Anglicize" guidelines later became McGill University, but the parameters of Canadian and Quebec education changed with the ever-mutable nature of the colonial (the colony would become "united" multiple times with the rest of the British North American colonies and change systems of government until 1867) and federal education guidelines and government recommendations.

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 believed that the colony of Lower Canada was in danger of being "lost to England and the Crown" around the early 1800s. Sewell was retained by the Governor of Lower Canada at the time (Sir Craig) to "analyze the political ills of the colony." Sewell believed and mentioned in this report that "the great links of connection between a Government and its subjects are religious, Laws and Language," and he was under the impression that "those links did not exist in the colony." Sewell claimed that the British and the Canadians (used to refer generally to people of British and/or French descent at this time) nurtured a "national antipathy" and that because "no incorporation of two such Extremes (as British and French mannerisms) can ever be effected." Sewell concluded that "the province (of Lower Canada) must be converted to an English Colony, or, it will ultimately be lost to England." Finally, Sewell spoke on why he thought these political ills arose in the colony in the first place: 1) "From the French predilections in the great Mass of the Inhabitants" (a fancy way of saying "to prefer the political traditions that many of the French colonists have within them for political rioting and revolution over loyalism) and 2) "From want of Influence and power in the Executive Government." This is also why Sewell sought out to control institutions that would influence the French population of Lower Canada at the time like the Roman Catholic Church and (when that failed) the education system through the Royal Institution for the Advancement of Learning (later McGill University) to increase the colonial anglicization of the colony.

He was born in Cambridge, Massachusetts, the son of Jonathan Sewall, the last British attorney general of Massachusetts and Esther Quincy. After a group of patriots attacked the family's residence, the Sewalls moved to Bristol, England; they adopted the spelling Sewell for the family name at this time. He attended Brasenose College, Oxford and then went to New Brunswick in 1785, where he studied law with Ward Chipman. He was named registrar of the Vice Admiralty Court for New Brunswick in 1787. In 1788, he was called to the bar and set up practice.

From 1799-1800 Sewell had conducted updated verses for "God Save the King" which caused somewhat of a sensation in 1800 when they were sung on stage in London by actor Richard Brinsley Butler Sheridan, after an assassination attempt on King George III.

In 1789, he moved to Quebec City from New Brunswick and qualified as a lawyer there. In 1790, he served as interim attorney general for the province, which after went to James Monk. 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.