There are 2 US Post Offices in Lamar County serving a total of 17,631 residents equating to 8,816 residents per post office. It's estimated that approximately 28,141 packages are handled across the 2 county post offices annually. Lamar County is a county located in the west central portion of the U.S. state of Georgia. As of the 2010 census, the population was 18,317. The county seat is Barnesville. Lamar County is included in the Atlanta-Sandy Springs-Roswell, GA Metropolitan Statistical Area."