The Susquehanna River forms the eastern border of the township with Northumberland County.

The census-designated place of Port Trevorton lies partially in the southern reaches of Union Township along the Susquehanna River and U.S. Route.

As of the census[4] of 2000, there were 1,519 people, 506 households, and 389 families residing in the township.

There were 530 housing units at an average density of 36.4 per square mile (14.1/km2).

19.0% of all households were made up of individuals, and 10.3% had someone living alone who was 65 years of age or older.