Union Township covers an area of 56.99 square miles (147.6 km2).

The population density was 24.28 inhabitants per square mile (9.37/km2).

The racial makeup of the township was 96.03% White, 1.70% Black or African American, 0.57% Native American, 0.19% Asian, 0.19% from other races, and 1.32% from two or more races.

Approximately 0.28% of the population were Hispanic or Latino of any race.

The township contains two cemeteries: Johnson and Pleasant Hill.