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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 km2) in area, with approximately 138 inhabitants per square mile (53/km2). As an unincorporated area, Monkton has no legally defined boundaries, and its ZIP code (21111) includes a portion of adjacent Harford County.
The community was named after Moncton, New Brunswick by Robert Cummings, a Pennsylvanian. Cummings initially named it Monckton Mills in honor of Rosanna Trites, a former love who was among the Pennsylvania Germans who had moved to Moncton in 1765. Cummings had also gone to Moncton, but returned in 1773 and settled at his uncle's estate in this community.
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. Also listed on the National Register are the Corbett Historic District and St. James Church.
The community was formerly served by the Northern Central Railway division of the Pennsylvania Railroad from the mid-19th century until the line's abandonment in 1972 and conversion to a hiking trail, the Northern Central Railroad Trail. The original Monkton station still remains.