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 that was not successful. The in, 1790, it then became part of the Territory South of the River Ohio  and finally, part of Tennessee after it was admitted to the Union on June 1,1796 as the 16th state. Jonesborough, the county seat of Washington County, is Tennessee's oldest town.

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