Hamilton County is a county located in the U.S. state of New York. It is named after Alexander Hamilton, the only member of the New York State delegation who signed the United States Constitution in 1787 and later the first United States Secretary of the Treasury. Its county seat is Lake Pleasant. It is one of only two counties that lie entirely within the Adirondack Park (Essex is the other). As of the 2010 census, the population was 4,836, making it the least populous county in New York. It is also the most sparsely populated county in the eastern half of the United States, with a population density of just over three people per square mile. (The easternmost county with a population density less than Hamilton County is Kenedy County in southern Texas.) There is no permanent traffic light in the county.
On April 12, 1816, Hamilton County was created by partitioning from Montgomery County, but due to low population it remained unorganized and administered from Montgomery County, N.Y. until it was recognized as sufficiently organized for self-government on January 1, 1838. The organization process was completed by Summer of 1847.
On April 6, 1860, Fulton County was partitioned, with of land in Sacandaga Park transferred to Hamilton County.
On May 24, 1915, land was swapped between Hamilton and Essex counties, with Hamilton ceding Fishing Brook Mountain for Indian Lake. Hamilton gained an additional , whereas Essex County lost . This left Hamilton with its present size of
The former town of Gilman was dissolved in 1860. The original county seat was Sageville, now part of Lake Pleasant.