It undertook a renowned winter march from Fredericton, New Brunswick, to Kingston, Ontario, in 1813 to defend Upper Canada from American invasion.

The unit participated in the 29 May Second Battle of Sacket's Harbor, an unsuccessful British attempt to capture a US naval base on Lake Ontario.

The flank companies were once again detached to fight in the 1814 Niagara campaign, seeing action at the inconclusive 25 July Battle of Lundy's Lane.

The regiment was present at the Siege of Fort Erie and took part in the unsuccessful assault of 15 August, during which their lieutenant-colonel, William Drummond, was killed.

[2] Fencibles were considered to be regular infantry units and came under the command of the British Army, rather than colonial control as was the case with the militia.

[8][9] Recruits were offered a bounty of five or six guineas and 200–500 acres (0.81–2.02 km2) of land in New Brunswick upon disbandment; a minimum height requirement of 5 feet 5 inches (1.65 m) was set.

[9][10] Recruiting parties were sent out in autumn 1803 and performed well in New Brunswick and Nova Scotia (these colonies would provide the majority of the enlisted personnel in the regiment).

[5][9] They also met with some success in Lower Canada, though they were refused access to Newfoundland by Governor Vice-Admiral Sir Erasmus Gower.

[5] The regiment passed its initial acceptance inspection in January 1806, the vast majority of the recruits being judged fit for service.

[14] Despite a temporary halt on recruitment implemented by governor of British North America Sir James Henry Craig in spring 1808 the regiment reached an enlisted strength of 63 sergeants, 26 drummers and 1,008 men in 1812.

The number was later reused for the 104th (Bengal Fusiliers) (1861–1881), after the British Army expanded with the incorporation of the regiments of the Honourable East India Company.

[17] As their terms of contract had changed all members of the fencibles regiment were required to re-attest to serve as regulars, all bar two enlisted men chose to do so.

[2] The 104th's uniform was modified after it changed status, for example the shoulderbelt plate was amended to show a "104" numeral surrounded by a crowned garter.

Later that year there were fears of an American invasion of Upper Canada and in the middle of winter it was decided to send the 104th from New Brunswick to reinforce the colony.

[12] The arrival of the reinforcements allowed the British commander George Prévost to launch an offensive on Sacket's Harbor, an American naval base on Lake Ontario.

[21] The regiment provided the majority of the British force engaged in the subsequent battle on 29 May 1813 and lost one-third of those committed to the unsuccessful attack.

[3] They reached there in time to join the end of the 24 June Battle of Beaver Dams and helped to guard American prisoners of war.

In an assault at 3 am on 15 August the regiment's light company succeeded in capturing a key bastion but was forced to retreat after a powder magazine exploded.

[21][12] Some 36 of the less-fit men, mainly those who had been stationed at Cape Breton and Prince Edward Island, were detached and transferred from the regiment in November 1814 into one of the Royal Veteran Battalions.

[23] The 104th is unique in Canadian history as the only regular British Army regiment to be raised and serve in the region during the Napoleonic period.

[20] In a ceremony in November 2014, the footbridge linking Carleton Street to the riverfront over St. Anne's Point Drive in Fredericton was officially named the New Brunswick’s 104th Regiment of Foot Bridge.