Monticello is a village located in the Town of Thompson in Sullivan County, New York, United States. The population was 6,512 at the 2000 census. It is the seat for the Town of Thompson and the county seat of Sullivan County. The village was named after the residence of Thomas Jefferson, but is pronounced as mon-tah-sell-oh. The Village of Monticello is in the central part of Thompson, adjacent to New York Route 17. Monticello is one of the largest villages in the county.