[3] Located in the southern part of the county, it borders the following townships: The village of New Knoxville is located in southwestern Washington Township.

According to the U.S. Census Bureau, the area of the township is 31.1 square miles (80.5 km2).

[5] Originally part of Allen County, the township was formed in 1836.

[6] The township is governed by a three-member board of trustees, who are elected in November of odd-numbered years to a four-year term beginning on the following January 1.

The northwestern section of the township is served by the Saint Marys (45885) post office, the northeastern section by the Wapakoneta (45895) post office, the southern section by the New Knoxville (45871) post office, and the extreme southeastern section by the Botkins (45306) post office.