MacNab was "likely the largest land speculator in Upper Canada during his time" as mentioned both in his official biography in retrospect and in 1842 by Sir Charles Bagot.

MacNab was left out of the regiment following regimental cuts after the War of 1812, and found employment in the law office of another Family Compact member's grandfather – George D'Arcy Boulton (aka D'Arcy Boulton Sr.)[1] He was born in Newark[2] (now Niagara-on-the-Lake) to Allan MacNab and Anne Napier (daughter of Captain Peter William Napier, R.N., the commissioner of the port and harbour of Quebec).

After the Queen's Rangers were disbanded, the family moved around the country in search of work and eventually settled in York (now Toronto), where MacNab was educated at the Home District Grammar School.

[4][1] In 1826, MacNab moved from York to Hamilton, where he established a successful law office, but it was chiefly by land speculation that he made his fortune.

[5] A successful entrepreneur as well as politician, MacNab, with Glasgow merchant Peter Buchanan, was responsible for the construction of the Great Western Railway of Ontario.

[1] 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.

MacNab was dispatched by Sir Bond Head on Christmas Day (25 December 1837) to command the troops in Niagara with support from both naval forces and regular officers.

MacNab saw himself alternating between "drilling or dining" for about 4 to 5 days as "supplies and billeting were inadequate and orders were vague" regarding command centers in Toronto and Montreal.

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 1829, MacNab refused to testify before a House of Assembly committee which was investigating the hanging of an effigy of Lieutenant Governor Sir John Colborne, chaired by Reformer W. W. Baldwin.

MacNab was subsequently sentenced to jail for 10 days by the House of Assembly, following apparent "prodding" from William Lyon Mackenzie.

MacNab returned to the public as a "Tory martyr", and effectively utilized/exploited this image to defeat the Reformers in Wentworth County and secure the political victory for both he and John Willson.

This was important as whoever controlled this office could "quietly acquire choice and undeveloped land in the Wentworth area without a need for a public auction".

[1] MacNab committed a breach of privilege and was arrested by the sergeant-at-arms during the 10th Parliament of Upper Canada after a motion by the legislative assembly.

This was beneficial for the Tories in Canada regarding their control of power in the Upper Canadian commercial and economic sectors, as MacNab acted as bridge for all members to communicate with each other, whereas previously there was only "intra-party maneuverings".

He married his second wife, Mary, who died 8 May 1846 and was a Catholic; she was the daughter of John Stuart, Sheriff of the Johnstown District, Ontario.

She married at Dundurn Castle, Hamilton, on 15 November 1855, William Keppel, Viscount Bury, afterwards the 7th Earl of Albemarle, who died in 1894.

One of her sons, the Honourable Derek Keppel, served as Equerry to The Duke of York after 1893 and was in Canada with His Royal Highness, in 1901 at 53 Lowndes Square, London, S. W., England.

The Toronto Globe and the Hamilton Spectator expressed strong doubts about the conversion, and the Anglican rector of Christ Church declared that MacNab died a Protestant.