Baltimore County is a county located in the northern part of the US state of Maryland. In 2010, its population was 805,029. It is part of the Baltimore-Washington Metropolitan Area. Its county seat is Towson. The name of the county was derived from the barony of the Proprietor of the Maryland colony, Cæcilius Calvert, 2nd Baron Baltimore, in County Longford, Ireland. Baltimore County does not include the city of Baltimore, which left the county to become an independent city in 1851.