Mount Sterling is a city in and the county seat of Brown County, Illinois, United States. The population was 2,006 at the 2020 census. Foodse…Mount Sterling is a city in and the county seat of Brown County, Illinois, United States. The population was 2,006 at the 2020 census. Foodservice distributor Dot Foods is based in Mount Sterling. Mount Sterling was organized in 1854. It did not have a courthouse until 1868, although it was designated the county seat from when the county was organized.