At the age of fifteen, Brock joined the 8th (The King’s) Regiment of Foot on 8 March 1785 with the rank of ensign, and was likely given responsibility for the regimental colours. His elder brother John was already an officer in the same regiment. As was usual at the time, Brock’s commission was purchased. On 16 January 1790 he bought the rank of lieutenant and later that year he raised his own company of men. As a result, he was promoted to captain (of an independent company of foot) on 27 January 1791 and transferred to the 49th (Hertfordshire) Regiment of Foot on 15 June 1791.

His nephew and biographer (Ferdinand Brock Tupper) asserts that shortly after Brock joined the regiment, a professional dueller forced a match on him. As the one being challenged Brock had his choice of terms, and he insisted that they use pistols. His friends were shocked as Brock was a large target and his opponent an expert shot. Brock, however, refused to change his mind. When the duellist arrived at the field, he asked Brock to decide how many paces they would take. Brock insisted that the duel would take place not at the usual range, but at handkerchief distance (i.e., close range). The duellist declined and subsequently was forced to leave the regiment. This contributed to Brock’s popularity and reputation among his fellow officers, as this duellist had a formidable reputation and was reportedly regarded as a bully in the regiment. During his time with this regiment, Brock served in the Caribbean, where he fell ill with fever and nearly died. He did not fully recover until after returning to England in 1793.

Once back in Britain he spent much of his time recruiting, and he was placed in charge of recruits on Jersey. He purchased his majority on 27 June 1795, and rejoined his regiment in 1796, when the rest of his men returned from the West Indies.

On 28 October 1797 Brock purchased the rank of lieutenant-colonel for £3,000, and became acting commanding officer of the regiment, assuming substantive command on 22 March 1798 with the retirement of Lieutenant Colonel Frederick Keppel. The rank was apparently bought cheaply; his predecessor from whom he purchased the rank was advised to sell up and leave the army rather than face a court martial and probable dismissal.

In 1799 the 49th was assigned to the Helder Expedition against the Batavian Republic (now known as the Netherlands), to be led by Sir Ralph Abercromby. During the troop landings, Brock saw his first combat on 10 September 1799 under the command of then-Major-General John Moore. Given that the 49th was in poor shape when Brock took command, they saw little actual combat. Likely Moore was sparing them and using more experienced troops to establish the beachhead. Finally on 2 October the 49th was actively involved in heavy combat at the Battle of Alkmaar, where they acquitted themselves well, sustaining only 33 deaths.

In 1801 while aboard the 74-gun HMS Ganges (commanded by Captain Thomas Fremantle, a personal friend), Brock was present at the Battle of Copenhagen. His troops were supposed to lead an assault on the forts at Copenhagen. The outcome of the sea battle made such an assault unnecessary, and Brock was able to observe first-hand the tactical brilliance of Lord Nelson. After the battle, Fremantle and Brock celebrated the victory with Nelson. In 1802 Brock and the 49th Foot were ordered to Canada.

Brock arrived in Canada with the rest of the 49th foot and was initially assigned to Montreal. Almost immediately, in 1804 he was faced with one of the primary problems in Canada: desertion. Seven soldiers stole a boat and fled across the river (and border) into the United States. Despite having no jurisdiction on American soil, Brock sent a party across the border in pursuit and the men were captured.

After a period of leave in England over winter 1805–6 and promotion to colonel on 29 October 1805, Brock returned to Canada temporarily in command of the entire British army there. By 1806 the United States was becoming increasingly hostile to the British Empire; relations between the two nations continued to deteriorate until war finally broke out in 1812.

In 1807 Brock was appointed brigadier general by Governor General Sir James Henry Craig, the new commander of Canadian forces. He was to take command of all forces in Upper Canada in 1810. During this time Brock continued to ask for a posting in Europe. In June 1811 he was promoted to major general and in October of that year Lieutenant Governor Francis Gore left for England. Brock was sent to Upper Canada as Senior Officer Commander of the Troops and senior member of the [Executive] Council, putting him fully in charge of both the military and civil authority. He was usually referred to as President of the Council or Administrator of Upper Canada (never as Lieutenant Governor). When permission to leave for Europe finally came in early 1812, Brock declined the offer, believing he had a duty to defend Canada in war against the United States.

Brock felt he needed to go further. He was hampered by Governor General George Prevost, who had replaced Craig in late 1811. Prevost’s orders from the government, and his own inclinations, were to emphasise defence. Prevost kept the bulk of his forces in Lower Canada to protect Quebec, and opposed any attack into United States. Brock also believed that he was handicapped by inertia and defeatism among the Legislature and other officials.

The United States declared war on Britain on 18 June 1812. Despite his preparations, Brock was worried about Canadian security. In Upper Canada, besides the militia, there was only one British regular infantry regiment, a detachment of retired veterans, and a company of artillery. These had to be dispersed among several widely separated posts. Brock’s advantage was that the armed vessels of the Provincial Marine controlled the lakes, and allowed him to move his reserves rapidly between threatened points.

Although Brock’s achievements in Canada were overshadowed by larger-scale fighting in Europe, his death was still widely noted, particularly in Guernsey. In London, he is remembered at a memorial in St Paul’s Cathedral. This was paid for by £1575 voted by the House of Commons, which also granted pensions of £200 to each of his four surviving brothers. For his actions in the capture of Detroit, Brock was appointed a Knight Companion of the Order of the Bath (KB) on 10 October 1812. He died at the Battle of Queenston Heights before learning of his knighthood.

Brock’s successor at Detroit, however, fared much worse. Colonel Henry Procter faced an attack from a resurrected American Army of the Northwest under future President William Henry Harrison. Harrison set out to retake Detroit, but a detachment of his army was defeated at Frenchtown on 22 January 1813. Procter, displaying poor judgement, left the prisoners in the custody of his First Nations allies, who proceeded to execute an indeterminate number of them. Subsequent American victories allowed Harrison to attempt another invasion of Canada, which led to the Battle of the Thames on 5 October 1813. After a successful American charge, Procter’s forces turned and fled, leaving Tecumseh and his American Indian troops to fight alone. They fought on, eventually being defeated. Perhaps of more importance to the British, at this battle Tecumseh died, and their alliance with the American Indians effectively ended.

As for Governor General Prevost, who often clashed with Brock, he remained in command of all British forces until after the Battle of Plattsburgh, in 1814. The battle was intended to be a joint naval/infantry attack, but Prevost did not commit his forces until after the naval battle had nearly ended. When he finally did attack, his forces proved unable to cross the Saranac River bridge, which was held by a small group of American regulars under the command of the recently promoted John E. Wool. Despite a heavy advantage in manpower, Prevost finally retreated upon hearing of the failure of the naval attack. For his failure at Plattsburgh, Prevost was recalled to England to face an inquiry, and a naval court martial determined that the blame for the loss at Plattsburgh primarily rested with Prevost. Prevost’s health failed him, and he died in early 1816.

In 1816, an unknown company issued a series of private half-penny tokens honouring Brock with the title “The Hero of Upper Canada”. Private copper tokens became common in Canada due to initial distrust of “army bills”, paper notes issued by Brock when there was a currency shortage caused by economic growth.

A small cairn at the foot of the Niagara Escarpment marks the spot where Brock fell. In 1824, Brock’s and Macdonell’s remains were moved into Brock’s Monument, which overlooked the Queenston Heights. That original monument was bombed and heavily damaged in 1840. (This action was reputedly by Irish-Canadian terrorist Benjamin Lett although a subsequent Assize failed to confirm this.). It was replaced by a larger structure 185 feet (56 m) high, built at public expense, that still stands. Brock’s remains were reinterred inside the new Monument on 13 October 1853. An inscription reads: “Upper Canada has dedicated this monument to the memory of the late Major-General Isaac Brock, K.B. provisional lieutenant-governor and commander of the forces in the province whose remains are deposited in the vault beneath. Opposing the invading enemy he fell in action near these heights on 13 October 1812, in the forty-third year of his age. Revered and lamented by the people whom he governed and deplored by the sovereign to whose services his life had been devoted.”

Since his death, several legends and myths about Brock have arisen. In 1908, the story of Brock’s betrothal to Sophia Shaw, the daughter of General Æneas Shaw, was first published. There is no supporting evidence for the claim and most biographers consider it apocryphal. A legend about Brock’s horse Alfred was first published in 1859. The horse was supposedly shot and killed during the battle while being ridden by Macdonell, and it is commemorated in a monument erected in 1976 in Queenston near the cairn marking the spot where Brock fell. But little evidence supports this account. The General’s horse “fully caparisoned, led by four Grooms,” is listed as preceding the coffin at the General’s interment at Fort George.

Brock University in Ontario provides scholarships to Guernsey students who achieve sufficiently high grades. In 1969, the Guernsey Post Office issued postage stamps to commemorate his life and achievements.

In September 2012, the Royal Canadian Mint issued a .99999 pure gold coin with a face value of 350 dollars to honor the bicentenary of Brock’s death. The reverse design was taken from a half-penny token issued in 1816 as a memorial to Brock. In addition, there have been quarters that have been released, one with a coloured maple leaf and the other with a frosted maple leaf.