He is best known to history for serving as both the civilian Governor General and the military Commander in Chief in British North America (now part of Canada) during the War of 1812 between Britain and the United States.

Prevost's maternal grandfather was a wealthy banker in Amsterdam, and his money is considered to have certainly been responsible for his grandson's quick advancement up the chain of command in the British Army, as promotion could then be obtained "by purchase".

On 1 January 1805, at the age of 37, Prevost was promoted to major-general, and soon after he was granted leave to return to England, where he became Lieutenant-Governor of Portsmouth and General Officer Commanding South-West District in December 1805,[3] and where he was appointed to be a baronet.

From 1808, to the beginning of the War of 1812, Prevost tried to encourage New England to trade with Britain by setting up "free ports" in Nova Scotia and New Brunswick, where American goods could be landed without a need to pay customs duties.

[1] The amendment of the law is considered to be a good achievement by Prevost, because his predecessor as lieutenant governor, John Wentworth, had been responsible for relations between the executive and legislative bodies of Nova Scotia weakening.

Eventually, Croke rejected the bill on the basis that it did not fit in with royal prerogatives, and then could not reach an agreement with the Legislative Council over how to settle the dispute between himself and the Assembly.

Prevost opposed Croke’s actions, restored "good understanding" with the Legislative Council, and then calmed the Assembly by deciding not to follow the constitution of Nova Scotia down to the letter.

[1] Prevost had become a popular lieutenant governor, but this was threatened by his attempts, beginning in 1810, to strengthen the Church of England in Nova Scotia, since that might alienate other religious groups.

In an effort to appease other religious groups, he appointed a number of Roman Catholic and Presbyterian clergy to be magistrates, and he authorized a grant of money for the Church of Scotland.

[1] During May 1811, when Prevost was preparing to oppose the Assembly over its policy of compensating its members for their expenses, feeling that it was irregular, open to abuse, and "an evil highly dangerous to the prerogative of the Crown", he was ordered to move to Lower Canada to replace Governor Craig.

On 21 October, he was appointed to be the governor-in-chief of British North America (Captain-General and Governor-in-Chief in and over the Provinces of Upper-Canada, Lower-Canada, Nova Scotia, and New-Brunswick, and their several Dependencies, Vice-Admiral of the same, Lieutenant-General and Commander of all His Majesty’s Forces in the said Provinces of Lower-Canada and Upper-Canada, Nova Scotia and New-Brunswick, and their several Dependencies, and in the islands of Newfoundland, Prince Edward, Cape Breton and the Bermudas, &c. &c. &c. Beneath Prevost, the staff of the British Army in the Provinces of Nova-Scotia, New-Brunswick, and their Dependencies, including the Islands of Newfoundland, Cape Breton, Prince Edward and Bermuda were under the Command of Lieutenant-General Sir John Coape Sherbrooke.

In 1812, Bédard, losing his motivation for continuing as leader, was given a judgeship in an area of British North America from which he could not have a major influence over the general political system.

In a report to the Colonial Office, Prevost said that he wanted to create a Legislative Council "possessed of the consideration of the country, from a majority of its members being independent of the government", in order to transfer to it "the political altercations which have been hitherto carried on by the governor in person.

[5] During the early months of 1813, Prevost twice visited Upper Canada where the military and civil situation was unsatisfactory after the Governor and Commander there (Major General Isaac Brock) had been killed in action.

A victory here could have been decisive but the expedition was hastily mounted and at the Battle of Sackett's Harbor both Prevost and the naval commander, Commodore James Lucas Yeo, attacked hesitantly.

Prevost had also made himself unpopular among some of the Army officers under his command who were veterans of the Peninsular War (such as Manley Power, Thomas Brisbane, and Frederick Philipse Robinson) by his perceived over-caution, and his niggling insistence on correct dress and uniform.

I see he has gone to war about trifles with the general officers I sent him, which are certainly the best of their rank in the army; and his subsequent failure and distresses will be aggravated by that circumstance; and will probably with the usual fairness of the public be attributed to it.

Later historians judge Prevost's preparations for defending the Canadas with limited means to be energetic, well conceived, and comprehensive, and against the odds he had achieved the primary objective of preventing an American conquest.

For although he was strategically inflexible and proved a hopeless field commander, as commander-in-chief facing overwhelming odds in the early years of the war he performed well; his sound political and administrative abilities formed the basis of success.