Carroll, county, northern Maryland, U.S. It consists of a piedmont region bounded by Pennsylvania to the north, the Patapsco River (north branch) and Liberty Reservoir to the southeast, the Patapsco River (south branch) to the south, and the Monocacy River to the northwest. The southeastern corner of the county includes part of Patapsco Valley State Park. Carroll county was formed in 1837 and named for Charles Carroll, a signer of the Declaration of Independence. The county seat is Westminster, a Union supply depot for the Battle of Gettysburg (July 1–3, 1863) during the American Civil War.
The main economic activities are manufacturing and agriculture (dairy cattle, oats, and corn [maize]). Area 449 square miles (1,163 square km). Pop. (2000) 150,897; (2010) 167,134.