It was established Jan. 3, 1815 and named Union as it was formed from parts of Deerfield and Turtlecreek townships.

The 2000 Census reported 4,668 residents, up slightly from 4,631 in 1990. Of those, 2,230 lived in the unincorporated part of the township and 2, 230 lived in the village of South Lebanon.

Union Twp. is governed by a three-member board of trustees.

Gravel mining is a major business. Interstate 71 crosses the township along with state routes 42 and 48. The city of Lebanon's sewage treatment plant is here, along the Little Miami River, a state and national scenic river.

Most of the township is in the Kings Local School District, which was formerly known as the Deerfield-Union Local School District, while parts are in the Lebanon City School District, the Mason City School District, and the Little Miami Local School District.