Jamestown is a borough in northwestern Mercer County, Pennsylvania, United States, along the Shenango River. The population was reported as 580 on the 2020 census. It is part of the Youngst…Jamestown is a borough in northwestern Mercer County, Pennsylvania, United States, along the Shenango River. The population was reported as 580 on the 2020 census. It is part of the Youngstown–Warren metropolitan area.