Michael O'Sullivan (May 4, 1784 – March 7, 1839) was a lawyer, militia officer, politician and judge in Lower Canada. Born in Clonmel, Ireland, he emigrated to Lower Canada as a child and was educated at the Collège Saint-Raphaël, where he began a life-long association with the Roman Catholic Sulpician order. He became a prominent member of the bar of Lower Canada, as well as a member of the Legislative Assembly of Lower Canada and Solicitor-general for Lower Canada.

O'Sullivan was an officer in the Lower Canada militia during the War of 1812. He participated in the Battle of the Châteauguay as aide-de-camp to Lieutenant-Colonel Charles de Salaberry, the commander of the Canadian and Mohawk forces who were defending Lower Canada from an invading American force. O'Sullivan was mentioned in dispatches for bravery after the battle, and subsequently wrote a contemporary account of the battle which has become the basis for understanding the battle.

O’Sullivan was elected to the Legislative Assembly of Lower Canada in 1814 and remained in office until 1824. He generally supported the Parti canadien, but gradually moved away from it as it became more nationalist. During his time as a member of the Legislative Assembly, O'Sullivan was vocal in his opposition to building a new, non-denominational, hospital in Montreal; his outspokenness on the matter culminated in his fighting a duel with a Montreal doctor, William Caldwell, who supported the proposal. The hospital was ultimately built.

In November 1838, O'Sullivan was appointed the Chief Justice of the Court of Queen's Bench in Montreal, but died in March 1839 at the age of 54.

O'Sullivan was born in Clonmel, Ireland, to John O'Sullivan and Eleonora O'Donel, who were related to prominent figures in County Tipperary. He emigrated to Lower Canada as a child, and was enrolled in the Collège Saint-Raphaël in Montreal from 1799 to 1806. The Collège was run by the Sulpician order, and his attendance began a life-long connection with the Sulpicians. He was recognised as a gifted student and finished at the top of his class. Other students in the class also went on to play leading roles in the nationalist politics of Lower Canada, such as Hugues Heney, André Jobin, and Jean-Moïse Raymond.^[1][2]

O'Sullivan was married twice. His first marriage, on June 1, 1809, was to Cécile Berthelet, daughter of the prosperous merchant and real estate owner, Pierre Berthelet, and his wife Marguerite Viger. One brother-in-law from his first marriage, Antoine-Olivier Berthelet, was briefly a member of the Legislative Assembly, from 1832 to 1834. Another brother-in-law, René-Joseph Kimber, was also a member of the Legislative Assembly. However, that marriage was short, ending with Cécile's death in 1811. He did not marry again for twenty years, until his second marriage to Jeanne-Marie-Catherine Bruyères, a widow, on May 17, 1831. Both marriages linked O'Sullivan to well-established members of the French-speaking community in Montreal.^[1][2]

During the War of 1812, O'Sullivan participated in the Battle of the Châteauguay, which saved Montreal from an invading American force. The battle took place at the junction of the Châteauguay River and the English River. The Canadian regular and militia units and Mohawk warriors were under the command of Lieutenant Colonel de Salaberry. They were outnumbered, but de Salaberry built up defensive positions with his forces protected by the dense woods, and trees and brushwood which they cut down. In a short battle, the Americans gave way and retreated, preserving Montreal from capture. The outcome of the battle had great strategic significance, since the capture of Montreal would have cut off Upper Canada from supply and communications with Lower Canada and Great Britain.^[1][2]

O'Sullivan was a lieutenant in a Beauharnois battalion of the Lower Canada militia, and served as aide-de-camp to de Salaberry. The Beauharnois battalions were heavily engaged in the battle. De Salaberry mentioned him in dispatches following the battle, praising O'Sullivan's bravery.^[1][2]

After the battle, a dispute arose about the role played by the Governor General, Sir George Prevost, and Major-General Louis de Watteville. Neither Prevost nor de Watteville had been present at the battle, but stories began to spread that the victory was due to their leadership. Lieutenant Colonel George Macdonell, who had been in command of the reserve at Châteauguay, also began to claim credit. In response, a pseudonymous letter appeared in the Montreal Gazette on November 9, 1813, written by a "témoin oculaire" ("eye-witness"), confirming the impressive role and command decisions played by de Salaberry, and asserting that the victory was due entirely to the Canadians.^[2] Subsequent research has indicated that the témoin oculaire was O'Sullivan. Some years later, Étienne-Paschal Taché, also a veteran of the battle, confirmed that O'Sullivan had written the letter. There was also an exchange of correspondence between O'Sullivan and de Salaberry shortly after the letter was published, which showed that he was the author. O'Sullivan's account of de Salaberry's role has become the accepted understanding of the battle.^[1][2][3][4]

O'Sullivan stayed in the militia after the War of 1812 was over. In 1830, he was promoted to major, on transferring to 1st Militia Battalion of Montreal.^[2]

After completing his courses at the Collège St-Raphaël, in 1805 O'Sullivan began his legal studies as an articled clerk with Denis-Benjamin Viger, a brilliant young lawyer and future joint-premier of the Province of Canada. He left Viger in 1808 and finished his articles under Stephen Sewell, another leading lawyer in Montreal. O'Sullivan may have made the switch to ensure he was trained in both the civil law under Viger, and the common law under Sewell. Knowing both systems would be an advantage for a lawyer in private practice in Montreal. He was called to the bar of Lower Canada in 1811.^[1][2]

O'Sullivan built a large practice in Montreal and became known as an authority on the law. He began to take in articling students, some of whom went on to distinguished legal careers themselves. In 1818, he was one of a team of lawyers retained to defend Lord Selkirk on charges in relation to the Pemmican War between the Hudson's Bay Company and the North-West Company. Between 1829 and 1831, O'Sullivan had received several public appointments, including the role of commissioner for receiving evidence, and as justice of the peace and King's Counsel. In 1831 and 1832, he served as president of the Advocates Library and Law Institute of Montreal, and gave lectures on topics such as the history of Roman law.^[1][2]

By the 1830s, O'Sullivan had gradually moved towards the Tory political viewpoint. His second wife's family and associates tended to the more conservative positions, and one of the witnesses to their marriage was Joseph Papineau, the conservative father of the Patriote firebrand, Louis-Joseph Papineau. In 1833, the Governor General appointed O'Sullivan to the position of solicitor-general of Lower Canada, citing O'Sullivan's "Public Estimation in this Province for probity, Professional ability & sound constitutional principles in politics."^[1][2]

In November 1837, as the unrest grew which would culminate in the Lower Canada Rebellion, O'Sullivan, along with Attorney General Charles Richard Ogden, wrote a legal opinion for Governor General Lord Gosford, which held that the Governor General had the power to declare martial law in the event of a rebellion. Gosford relied on that opinion when he declared martial law in the district of Montreal on December 5, 1837.^[9]

In November 1838, O'Sullivan was appointed Chief Justice of the Court of Queen's Bench for the district of Montreal, but only served for a single term in February 1839. He died on March 7, 1839, aged 54. An autopsy found Caldwell's ball from the duel 20 years prior lodged against the middle of O'Sullivan's spine. He was buried from Notre-Dame in Montreal.^{[1][2][7][10]}