Washington County is a county located in the U.S. state of Tennessee. As of the 2010 census, the population was 122,979. Its county seat is Jonesborough. The county’s largest city and a regional educational, medical and commercial center is Johnson City. Washington County is Tennessee’s oldest county, having been established in 1777 when the state was still part of North Carolina. Washington County is part of the Johnson City, TN Metropolitan Statistical Area, which is a component of the Johnson City–Kingsport–Bristol, TN-VA Combined Statistical Area, commonly known as the “Tri-Cities” region.