Barnesville is a city in Lamar County, Georgia, United States. The city is a part of the Atlanta Metropolitan Area. As of the 2010 census, the city had a population of 6,755. The city is the county seat of Lamar County.Barnesville was once dubbed the "Buggy Capital of the South" as the town produced about 9,000 buggies a year around the turn of the 20th century. Each year in the third week of September the town hosts an annual Buggy Days celebration.