The county consists of several non-contiguous and sparsely populated islands and peninsulas of Lake Champlain, connected to each other by U.S. Route 2.

[5][6][7] The land was originally contested by Massachusetts, New Hampshire, New France and New Netherland, 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.

Vermont's border with Quebec was established at 45 degrees north latitude,[11][12] which explains why this county has no dry-land connection to the rest of the United States.

[13] New York, still not satisfied with the relinquishment of its land to Vermont, asked the U.S. Congress to arbitrate the matter.

This commission ruled before Vermont's admission, which took place on March 4, 1791, but a small change they permitted has never been acted upon.

[4] In the late 19th century the Rutland Railroad ran service from northern New York State by the Canada–U.S.

From 1899 a series of causeways provided continuous train service north–south through the Lake Champlain islands, making a direct connection to Burlington.

Four of its five towns (North Hero, South Hero, Grand Isle and Isle La Motte) are situated entirely on islands in Lake Champlain, while Alburgh is on a peninsula (an exclave known as the Alburgh Tongue) extending south into the lake from Quebec.

Alburgh is the physically largest landmass of Grand Isle County, and it lies on a peninsula that borders Quebec to the north.

[25] In terms of ancestry, 18.0% were English, 14.2% were Irish, 10.6% were French Canadian, 10.3% were German, 10.3% were American, and 6.9% were Scottish.

From William Henry Harrison in 1836 to Winfield Scott in 1852, the county voted for Whig Party candidates.

From John C. Frémont in 1856 to William Howard Taft in 1908, the Republican Party had a 52-year winning streak in the county.

Republican candidates Warren Harding, Calvin Coolidge and Herbert Hoover won the county in 1920, 1924 and 1928, respectively.

Democratic candidate Franklin D. Roosevelt won Grand Isle County in all four of his presidential runs, from 1932 to 1944.

During that time, Grand Isle, Chittenden and Franklin Counties became Democratic enclaves in an otherwise Republican-voting Vermont.