Monkton is an unincorporated community in northern Baltimore County, Maryland, United States. It has a population of about 4,856 people. It is 35 square miles (91 km) in area, with approximately 138 inhabitants per square mile (53 /km). As an unincorporated area, Monkton has no legally-defined boundaries, and its postal ZIP code (21111) includes a portion of adjacent Harford County. East of Monkton is an area named "My Lady's Manor", known for its horse farms, sprawling countryside, and old, stately homes set back from the country roads. In 1713, Charles Calvert, 3rd Baron Baltimore, decreed 10,000 acres (40 km²) for himself. He made a gift of this land to his fourth wife, christening the estate "My Lady's Manor. " It was added to the National Register of Historic Places in 1978.