Hamilton County is a county in the U.S. state of New York. As of the 2010 census, the population was 4,836, making it the least populous county in New York. With an area nearly the size of Delaware, but with a population in 2019 of only 4,416, it is the least densely populated county east of the Mississippi River. Its county seat is Lake Pleasant. The county 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. The county was created in 1816 and organized in 1847.