Washington Township was founded circa 1809-1810 out of portions of Darby, Franklin and Liberty townships. The town of Dublin was established on the west bank of Scioto River in 1810, and remained a small village until the 1980s, when corporations took notice of the high standards of Washington Township and the Village of Dublin, and began locating in the community.

Washington Township serves portions of three counties: Delaware, Franklin, & Union. In the early 1990s, the City of Dublin found itself being serviced by four different township fire departments in three counties. Under state laws in effect at the time, officials later transferred the city land in Concord Township in Delaware County to Washington Township, where the majority of the city was located. Dublin transferred other city-annexed lands into Washington Township as well. Those include land located in Jerome Township, Union County; and Perry Township, in Franklin County; thereby establishing one service provider for the entire city.

Currently, the Township provides services to unincorporated areas in Franklin County as well as the City of Dublin. The unincorporated areas in Franklin County are located between the Scioto River and Madison County. Washington Township's unincorporated areas are largely farmland and single family houses in the western portion of the township, primarily between Cosgray Road and Madison County. Other areas include the town of Amlin, and smaller tracts in the vicinity of Hayden Run and Dublin Road.

Washington Township is among the top 20 townships in the state in terms of population