Richmond County was established in 1777 by the first Constitution of the (newly independent) State of Georgia. As such, it is one of the original counties of the state. It was formed out of a portion of the colonial Parish of St. Paul, after the Revolution disestablished the Church of England in the (former) Royal Province of Georgia.
Source - Wikipedia
Columbia, McDuffie, Jefferson and Burke counties as well as the South Carolina counties of Edgefield and Aiken.