Washington Township was established in 1846 after inhabitants in Mifflin Township asked Dauphin County for a division of their township because it was too large to maintain. Washington Township is a scenic area of fertile farmland, mountains and residential areas. Washington Township is home to the only remaining covered bridge (Henninger Farm Bridge) in Dauphin County. The Henninger Farm Bridge was destroyed by fire, but was rebuilt in 2003, restoring it to all of its original splendor; known for the Burr Arch design. The covered bridge is registered with the National Register of Historical Places. Washington Township includes the unincorporated community of Loyalton, also known as Oakdale Forge and Oakdale Station at various time in history.
Seven of the original nine one-room schools still remain standing. A significant portion of the Wiconisco Creek, known to the local residents as the Black Creek flows through the township.