[3] According to the United States Census Bureau, the township has a total area of 36.1 square miles (93 km2), all land.

Among previously existing settlements in Washington Township is Pack's Mills.

This small community on the Black River was named after George Pack, Jr. (brother of John Pack), the owner of the sawmill that gave the town its name.

There were 670 housing units at an average density of 18.5 per square mile (7.2/km2).

18.7% of all households were made up of individuals, and 8.5% had someone living alone who was 65 years of age or older.