Brown County was formed in 1839 from the southern portion of Schuyler county. It was named for Jacob Jennings Brown, a United States General, who came to the attention of the nation during the War of 1812. At the time of the most recent census the population of Brown County numbered less than 6,000. Mt. Sterling is the county seat. Other towns in Brown county include Cooperstown, Timewell, Ripley and Versailles.
The following links will provide more information.
Brown County History & Maps
Illinois County Formation and Census Maps
You can get a GNIS List of Places anywhere in the US (Townships, cemeteries, schools, etc.) by name or other characteristic.
Mount Sterling in 1867 - Map at Library of Congress
Brown County in the Civil War
The Military Tract
Birds-eye View of Mount Sterling - 1869
Brown County Main Page