Pen Mar is an unincorporated community and census-designated place (CDP) in Washington Township, Franklin County, Pennsylvania, United States. It is located along the Mason–Dixon line, bordered to the south by the community of Pen Mar Park in Maryland. As of the 2010 census the population of Pen Mar was 929.GeographyPen Mar is located in southeastern Franklin County, Pennsylvania, on the western slope of South Mountain. Pennsylvania Route 550 is the main road through the community, leading northwest down the mountain 2mi to Rouzerville. Waynesboro, Pennsylvania, is northwest of Pen Mar, and the site of the former Fort Ritchie in Maryland is to the southeast.