Georgia's 50th county, Henry was created by the Georgia State Legislature in 1821 from land acquired from the Creek Indian Nation by the First Treaty of Indian Springs. It was named for Patrick Henry, whose powerful rhetoric helped push the Colonies into the Revolutionary War. The county was created in May 1821 by the Georgia Land Lottery Act of that year. Enacted four months after the Creeks ceded a large portion of their land in the Treaty of Indian Springs, the act named five new counties, Henry among them.
Henry's original land area was much larger than it is today, stretching from near Indian Springs (present-day Indian Springs State Park) in the south to the Chattahoochee River near Sandy Springs in the north; encompassing most of present day Metropolitan Atlanta. Before one year passed the size of the County was diminished through the separation of land areas which, in whole or in part, became present day DeKalb, Fulton, Fayette and Newton Counties. Later divisions resulted in Clayton, Spalding, Rockdale and Butts Counties.
Source: georgia.gov and Wikipedia - http://en.wikipedia.org/wiki/Henry_County,_Georgia
Rockdale, DeKalb, Newton, Butts, Spalding and Clayton Counties.