Marietta is a borough in Lancaster County, Pennsylvania, United States. The population was 2,633 at the 2020 census. It is located on the east bank of the Susquehanna River, northwest of Columbia. Marietta is located in western Lancaster County at 40°3′26″N 76°33′21″W / 40.05722°N 76.55583°W. It is bordered to the north, east, and west b…Marietta is a borough in Lancaster County, Pennsylvania, United States. The population was 2,633 at the 2020 census. It is located on the east bank of the Susquehanna River, northwest of Columbia. Marietta is located in western Lancaster County at 40°3′26″N 76°33′21″W / 40.05722°N 76.55583°W. It is bordered to the north, east, and west by East Donegal Township, and to the south, across the Susquehanna River, by Hellam Township in York County.