Hamilton County, New York: History
On April 12, 1816, Hamilton County was created by partitioning 1,800 square miles (4,700 km2) from Montgomery County, but due to low population it remained unorganized and administered by Montgomery County until it was recognized as sufficiently prepared for self-government on January 1, 1838. The organization process was completed by summer 1847.
On April 6, 1860, Fulton County, which had been partitioned April 18, 1838, had 10 square miles (26 km2) 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 20 square miles (52 km2), whereas Essex County lost 30 square miles (78 km2). This left Hamilton with its present size of 1,830 square miles (4,700 km2).
The former town of Gilman was dissolved in 1860. The original county seat was Sageville, now part of Lake Pleasant.