Carroll County was created in 1837 from parts of Baltimore and Frederick Counties. The County was named for Charles Carroll of Carrollton. Carroll was a Marylander and the last surviving signer of the Declaration of Independence. He died in 1832 at the age of 95. During the American Civil War, the population of Carroll County was sharply divided between supporters of the Union and the Confederacy. In 1863, there were historic troop movements through the county as part of the Gettysburg campaign. On June 29, 1863, the cavalry battle of Corbit's Charge was fought in the streets of Westminster. This would later be recognized as a contributing factor in the eventual defeat of Robert E. Lee's army at Gettysburg.

Carroll County is bordered on the north by the Mason-Dixon Line. The Patapsco River forms the southern border and Liberty Reservoir forms part of the eastern border. Carroll County is bordered on the west by the Monocacy River and Sam's Creek.