Counties were among the earliest units of local government established in the Thirteen Colonies that would become the United States. Virginia created the first counties in order to ease the administrative workload in Jamestown. The House of Burgesses divided the colony first into four "incorporations" in 1617 and finally into eight shires (or counties) in 1634: James City, Henrico, Charles City, Charles River, Warrosquyoake, Accomac, Elizabeth City, and Warwick River. America's oldest intact county court records can be found at Eastville, Virginia, in Northampton (originally Accomac) County, dating to 1632. Maryland established its first county, St. Mary's, in 1637, and Massachusetts followed in 1643. Pennsylvania and New York delegated significant power and responsibility from state government to county governments and thereby established a pattern for most of the United States, although counties remained relatively weak in New England.
When independence came, "the framers of the Constitution did not provide for local governments. Rather, they left the matter to the states. Subsequently, early state constitutions generally conceptualized county government as an arm of the state." In the twentieth century, the role of local governments strengthened and counties began providing more services, acquiring home rule and county commissions to pass local ordinances pertaining to their unincorporated areas.
In some states, these powers are partly or mostly devolved to the counties' smaller divisions usually called townships, though in New York, New England and Wisconsin they are called "towns". The county may or may not be able to override its townships on certain matters, depending on the state constitution.
The newest county in the United States is the city and county of Broomfield, Colorado, established in 2001 as a consolidated city-county. The newest county-equivalents are the Alaskan boroughs of Skagway established in 2007, Wrangell established in 2008, and Petersburg established in 2013.