The area of present day Ulster County was called Esopus by the first European settlers. In 1652 Thomas Chambers a freeholder from Rensselearwyck purchased land at Esopus and began trading there. In 1683, the Duke of York created twelve counties in his province. Ulster County was one of them. Its boundaries at that time included the present Sullivan County, and portions of the present Delaware, Orange, and Greene Counties.
In 1777, the capital of New York State (the first state capital of independent New York) was established at Kingston, though it was subsequently moved to Wawarsing when the British burned that city.
In 1797, portions of Otsego and Ulster Counties were split off to create Delaware County.
In 1798, the southernmost towns in Ulster County were moved into Orange County, to compensate Orange for breaking away the southernmost portion of that county in order to form Rockland County.
In 1800, portions of Albany and Ulster Counties were split off to create Greene County.
In 1809, Sullivan County was split off from Ulster County.
During the American Civil War volunteers were recruited from the more affluent families of the County to form the 139th New York Volunteer Infantry Regiment.