Monongalia County, known locally as Mon County, is a county located in the U.S. state of West Virginia. As of the 2020 census, the population was 105,822, making it West Virginia's third-most populous county. Its county seat is Morgantown. The county was founded in 1776. Monongalia County is included in the Morgantown, WV Metropolitan Statistical Area, and is the largest county in North-Central West Virginia. It is part of the Pittsburgh media market.