Upper Canada Rebellion, 1837
Before the Rebellion broke out, MacNab argued for increased American immigration as "they are a useful and enterprising people and if admitted would be of great advantage to the country" in 1837. Again before the Rebellion, MacNab was appointed as Lieutenant-Colonel of the 4th Regiment of the Gore militia in May 1830, partly through the influence of the Chisholm family of Oakville.
MacNab opposed the reform movement in Upper Canada that was led by William Lyon Mackenzie. When Mackenzie led the Upper Canada Rebellion in 1837, MacNab was part of the force of British regular troops and Upper Canada militia that moved against Mackenzie at Montgomery's Tavern in Toronto on 7 December, dispersing Mackenzie's rebels in less than an hour.
Following the rebels' defeat at Montgomery's Tavern, Charles Duncombe led an uprising at London. Due to his leadership at Montgomery's Tavern, Lieutenant Governor Sir Francis Bond Head gave MacNab sole command of troops sent to London. He also led a militia of his own against the rebels. Duncombe's men also dispersed themselves when they learned that MacNab's forces were waiting for them.
Despite these twin successes, some regarded the quality of MacNab's leadership as "mixed". He had had "extreme problems" in communication, procuring supplies, and controlling the volunteers, and had ignored basic operational procedures. MacNab was given 250 troops but ultimately had some 1500 men assembled total, as MacNab argued "as early as December 14".
Mackenzie fled to the United States following his defeat at the Battle of Montgomery's Tavern on 7 December, and returned to Canada on 13 December, occupying Navy island, with strong American sympathy. Sir Bond Head dispatched MacNab on Christmas Day (25 December 1837) to command the troops in Niagara with support from both naval forces and regular officers of British units. MacNab alternated between "drilling or dining" for about 4 or 5 days as "supplies and billeting were inadequate and orders were vague" regarding command centers in Toronto and Montreal. Moreover, contradictory reports came to both Head and MacNab regarding the amount of U.S. supplies and the strength and morale of Mackenzie's new rebel force. Head refused to sanction n attack on Navy island but offered no other alternatives. Some 2000 raw and reckless volunteers amassed as troops by 29 December.
29 December proved to be important to MacNab as two attacks happened against Mackenzie's forces on 29 December under MacNab's command: a dawn attack and a dusk attack. The first attack proved how little discipline the officers had under MacNab, how little control MacNab had over them, and how weak the line of command was. The second attack showed how reckless MacNab could be if his position as "commander" was questioned.
The dawn (first) attack was not sanctioned or ordered by MacNab and was the result of actions taken by a group of bibulous officers. The attack nearly ended in disaster.
The second (dusk) attack, done at MacNab's order, did end in disaster. MacNab and Captain Andrew Drew, a retired officer of the Royal Navy, commanding a party of militia, acting on information and guidance from Alexander McLeod, crossed into New York and destroyed Mackenzie's supply ship. The sinking of the SS Caroline happened in U.S. waters. A U.S. citizen was killed in the action. The official reaction was swift and immediate. The event, known as the Caroline affair, caused MacNab indicted for murder in Erie County, New York, He was replaced by Colonel Hughes, who took MacNab's post of Commander in Niagara. However, before leaving the frontier MacNab protested that Hughes would be the one to receive "all the credit" whilst MacNab and the militia had done "all the drudgery". Later, MacNab quitted the Niagara frontier on 14 January 1838. By them some 3500 troops were amassed to fight the rebels. On the evening of 14 January, Mackenzie and his force slipped off the island, and while Hughes (MacNab's replacement) was occupying Navy island, MacNab was lobbying in Toronto for his command position back.
During the Rebellions, MacNab appreciated that rebels had had varying "degrees of involvement" with rebellion. He punished "only the rebel leaders" under his own initiative, and he saw the "common followers" of the rebels as people who were "deceived", and even promised clemency to some. MacNab also shared a common philosophy in his own troops, believing that officers earn the respect of their subordinates "not only through courage in war but also by tempering strict justice with kindness and approachability off the battlefield".
In 1838, Macnab was knighted for his zeal in suppressing the rebellion.
In 1860, Macnab was appointed an honorary colonel in the British army, and aide-de-camp to Queen Victoria.
