By 1811 a post office had been established in the village. Two years later, the Ashby's Gap Turnpike was completed from Aldie to Middleburg, and in 1818 the Snickersville Turnpike opened, replacing the Mountain Road. By the census of 1820, Aldie had a population of 248 residents, making it the fourth largest town in the county. The population peaked in 1830 at 260—notably more than half, 132, were slaves. With the incorporation of Middleburg the following year, Aldie began a slow decline. During the Civil War, the village itself and lands immediately to the west and northwest were the site of the Battle of Aldie during the Gettysburg Campaign. In addition, the Confederate partisan John Singleton Mosby was active in the village, and several small skirmishes between Union cavalry and his band of rangers took place in and around Aldie.