Barnesville is a city in Lamar County, Georgia, United States. The city is a part of the Atlanta Metropolitan Area. The most populated city named Barnesville in the United States. The city is the county seat of Lamar County. The city has a total area of 5.7 square miles (15 kilometer), of which, 5.7 square miles (15 kilometerm) of it is land and 0.18% is water. Barnesville was once dubbed the "Buggy Capital of the World" as the town produced about 16,000 buggies a year around the turn of the century. Each year in the third week of September the town hosts an annual Buggy Days celebration.