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 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.
Washington County is rooted in the Watauga settlements, which were established in the early 1770s in the vicinity of what is now Elizabethton, in adjacent Carter County. At the outbreak of the Revolutionary War in 1776, the Wataugans organized the "Washington District," which was governed by a committee of safety. North Carolina initially refused to recognize the settlements as legal, but finally agreed to annex the district after the settlers thwarted an invasion by hostile Cherokees. The settlements were governed as the Washington District, which originally included all of what is now Tennessee. The district was reorganized as Washington County in 1777.
From 1784 through 1788, the county was part of the State of Franklin, an early attempt to create a fourteenth state prior to Kentucky and Vermont's admissions into the union. It then became part of the Southwest Territory in 1790, and finally, part of Tennessee after it was admitted to the Union in 1796 as the 16th state. Jonesborough, the county seat of Washington County, is Tennessee's oldest town. With many buildings restored, it comprises one of the nation's most authentic historic districts of the period 1790–1870.
Washington County was divided between pro-Union and pro-secession sentiments at the outset of the Civil War. In Tennessee's Ordinance of Secession referendum on June 8, 1861, Washington Countians voted 1,445 to 1,022 in favor of remaining in the Union. One of the bridges targeted by the East Tennessee bridge-burners in November 1861 was located in what is now Watauga near the Washington-Carter county line. Landon Carter Haynes, a Confederate senator, hailed from Washington County.
Johnson City, originally known as Johnson's Depot, was a major railway center for the southeastern states, connecting the region for freight transportation and passengers. It was the headquarters for both the standard-gauge Carolina, Clinchfield, and Ohio (Clinchfield Railroad), which required the excavation and blasting of 17 tunnels during its construction; and the narrow-gauge East Tennessee and Western North Carolina Railroad (Tweetsie). Significant restoration is underway, as well as publicizing the railroad heritage of the Johnson's Depot Historic District. Other historic properties are being restored as representative of Johnson City's late nineteenth and early twentieth-century era as a railway center.