[5][6][7] The land originally was contested by Massachusetts, New Hampshire, and New York, but it remained undelineated until July 20, 1764, when King George III established the boundary between New Hampshire and New York along the west bank of the Connecticut River, north of Massachusetts and south of the parallel of 45 degrees north latitude.

On September 3, 1783, as a result of the signing of the Treaty of Paris the Revolutionary War ended with Great Britain recognizing the independence of the United States.

Article II of the treaty agreed on boundaries between the United States and British possessions to the north, and included Vermont within the U.S. Vermont's border with Quebec was established at 45 degrees north latitude.

[4] However, Vermont's government continued to take the position that it was independent of both the United States and Britain, and so it remained until 1791.

In 2008, the federal government declared the county a disaster area after severe storms and flooding on June 14–17.

[15] As of the 2010 United States Census, there were 47,746 people, 18,513 households, and 12,939 families residing in the county.

In 1836, the county was won by Democratic Party candidate Martin Van Buren From William Henry Harrison in 1840 to Winfield Scott in 1852, the county would vote the Whig Party candidates.

Dwight D. Eisenhower was able to win back Franklin County for the Republicans during the 1952 and 1956 elections.

The county would go to Democratic candidates John F. Kennedy in 1960, Lyndon B. Johnson in 1964, and Hubert H. Humphrey in 1968.

Incumbent President Richard Nixon would carry the county in 1972 as would Gerald Ford in 1976.