[2] The township is located in the northeastern corner of Butler County.

The southwestern end of the Interstate 80 bridge over the Allegheny River is in Allegheny Township, but the interstate highway curves back to the north out of the township 2 miles (3 km) to the west.

According to the United States Census Bureau, the township has a total area of 24.6 square miles (63.8 km2), all land.

The racial makeup of the township was 98% White, 1.6% African American, 0.2% Asian, and 0.2% from two or more races.

24.0% of all households were made up of individuals, and 10.4% had someone living alone who was 65 years of age or older.