Marshall County is a county located in the U.S. state of West Virginia. At the 2010 census, the population was 33,107. Its county seat is Moundsville. Its southern border is the Mason-Dixon line.Marshall County is included in the Wheeling, WV-OH Metropolitan Statistical Area.Marshall County is home to the largest conical burial mound in North America, at Moundsville. Marshall County was formed in 1835 from Ohio County by act of the Virginia Assembly.