According to the U.S. Census Bureau, the county has a total area of 997 square miles, of which 968 square miles is land and 29 square miles (2.9%) is water.
Sullivan County is in the southern part of New York State, southeast of Binghamton and southwest of Albany. It is separated from Pennsylvania along its southwest boundary by the Delaware River.
The county, which starts about 70 miles northwest of New York City, is in the Catskill Mountains. Its northeastern corner is within the Catskill Park.