The present border originated with the Treaty of Paris in 1783, which ended the war between Great Britain and the separating colonies which would form the United States. The Jay Treaty of 1794 created the International Boundary Commission, which was charged with surveying and mapping the boundary. Disputes over the interpretation of boundary demarcation led to the Aroostook War and the ensuing Webster–Ashburton Treaty in 1842 which better defined the boundary between Maine and New Brunswick and the Province of Canada. Westward expansion of both British North America and the United States saw the boundary extended west from the Lake of the Woods to the Rocky Mountains under the Convention of 1818. An 1844 boundary dispute during U.S. President James K. Polk's administration led to a call for the northern boundary of the U.S. to be 54°40′north (related to the southern boundary of Russia's Alaska Territory), but the United Kingdom wanted a border that followed the Columbia River to the Pacific Ocean. The dispute was resolved in the Oregon Treaty of 1846, which established the 49th parallel as the boundary through the Rockies. After the Pig War in 1859, the San Juan Islands were given to the United States. In 1903 a joint United Kingdom – Canada – U.S. tribunal established the boundary with Alaska. In 1925 the International Boundary Commission was made a permanent organization responsible for surveying and mapping the boundary, maintaining boundary monuments (and buoys where applicable), as well as keeping the boundary clear of brush and vegetation for 6 metres (20 ft) on each side of the line.

Commonly referred to as the world's longest undefended border, the International Boundary is actually defended, but by law enforcement and not military personnel. The relatively low level of security measures stands in stark contrast to that of the United States-Mexico border (1/3 as long as the Canada-U.S. border), which is actively patrolled by U.S. customs and immigration personnel to prevent Mexican citizens and other migrants from illegally entering the United States (see United States-Mexico barrier).

After the September 11, 2001 terrorist attacks in the United States, border security along the International Boundary was dramatically improved by both nations in both populated and rural areas. Both nations are also actively involved in detailed and extensive tactical and strategic intelligence sharing. It is a common misconception that the 19 terrorists involved in the September 11th attacks entered the United States via the Canadian border.**

American and Canadian residents owning property adjacent to the border are required to report construction of any physical border crossing on their land to their respective governments, and this is enforced by the International Boundary Commission. Where required, fences or vehicle blockades are used. All persons crossing the border are required to report to the respective customs and immigration agencies in each country. In remote areas where staffed border crossings are not available, there are hidden sensors on roads and also scattered in wooded areas near crossing points and on many trails and railways, but there are not enough border personnel on either side to verify and stop coordinated incursions (see the Michel Jalbert story)

Parts of the International Boundary cross through mountainous terrain or heavily forested areas, but significant portions also cross remote prairie farmland and the Great Lakes and Saint Lawrence River, in addition to the maritime components of the boundary at the Atlantic, Pacific, and Arctic oceans. The border runs through the middle of the Akwesasne Nation. The actual number of U.S. and Canadian border security personnel is not known but estimated to be less than 1,000 in total, largely clustered near major crossing points. In comparison, there are in excess of 7,000 U.S. border security personnel on the Mexico-U.S. border alone.

In past years Canadian officials have complained of cigarette and firearms smuggling from the United States while U.S. officials have complained of drug smuggling from Canada. Human smuggling into both countries has been an ongoing problem for border security and law enforcement personnel, although a minor one in comparison to the Mexico-U.S. border. In July 2005 law enforcement personnel arrested three men who had built a 100 meter (330 ft) tunnel under the border between British Columbia and Washington that they intended to use for smuggling marijuana, the first such tunnel known on this border.

In recent years there has been a voluntary border patrol along the Canadian border of Vermont. This is not a government funded cause. Some see more comedy than security, such as when The Daily Show interviewed several of its members.