In 1787, the fledgling United States defined the Northwest Territory, which included the area of present-day Indiana.

Starting in 1794, Native American titles to Indiana lands were extinguished by usurpation, purchase, or war and treaty.

The United States acquired land from the Native Americans in the Treaty of Grouseland (1805), by which a large portion of the southern Indiana Territory became property of the government.

Interim commissioners were named and directed to determine the proper choice of the seat of government.

Accordingly, they began deliberating in January 1814, and by February 2 had selected an uninhabited site near the center, naming it 'Salem'.

[5] In the territorial act creating the county, it was named for U.S. President George Washington,[6] who had died fourteen years earlier.

[9][10] By 1850, 252 Black people had settled in the county, mainly living in Posey and Washington townships.

[10] Whitecapping, the process by which rural citizens used threats or extralegal violence to force Black people out of the region, continued in Washington County during the Civil War.

In December 1864, John Williams, a prosperous Black farmer in the county, was shot dead in the doorway of his home.

[16] Law enforcement would not allow Black people to stop in Salem, and would escort them to the county line.

[20] Washington County remained sundown until 1990 at the latest, when 15 Black people were recorded living in Salem on that year's census.

[20][16] The low rolling hills of Washington County were tree-covered before settlement, but have been largely cleared and devoted to agriculture, although drainage areas are still wooded.

The East Fork of the White River joins the Muscatatuck near the center of the county's north line.

The highest point on the terrain (1,050 feet (320 meters) ASL) is an isolated rise two miles (3.2 km) NNW from New Philadelphia in the eastern part.

[25] Four were found dead in a home on Old Pekin Road according to Washington County officials.

The commissioners execute the acts legislated by the council, collect revenue, and manage the county government.

[29] Washington County is part of Indiana's 9th congressional district and is represented in Congress by Republican Erin Houchin.