Allen Township is located in the southwestern portion of Union County, bounded by Liberty and Paris Twps. from the north, Paris Twp. from the east, Union Twp. and Champaign Co. on the southern borders, and Logan Co. from the west. County Commissioners formed Allen Twp. on June 5, 1827, the fifth in date of organization in Union County. Southern townships were settled sooner, but Allen Twp. was settled rapidly in a 15 year time period once families began moving to the area.

Daniel Allen came to Madison Co., Ohio about 1818, and then moved to Allen Twp. about 1820-21. He was one of the earliest known settlers, as no one knows exactly who were the very first settlers. Daniel Allen was highly respected, was one of the very first 13 citizens to vote in the township, and was one of the first trustees. He was killed by falling from a load of oats and broke his neck in 1832. The township was named in his honor. Other known early settlers were: Henry Vangordon (first trustee, fourth treasurer), Isaac Allen (brother to Daniel Allen), Jonathan Burwell, Southard Mather (first clerk), Moses Redford, William Epps, William W. Haines, Jesse Haines (brother to William W.), William Asher, John Asher (son of William), William Milligan, Samuel Milligan (brother to William), Nimrod Garwood (first trustee, first treasurer and clerk), Joseph Russell, John Paver, Abraham Leonard, Hollis Amy, Amos A. Williams, Alvin Wilcox, Luthur Wood, Matlock Stokes, William Snuffin, William Inskeep, Jr., Elisha Cowgill, Edward Tyler, Cornelius Meshon, John Milligan, John Dawson, Joseph Dunlap, Daniel Spain, Abraham Holycross, Isaac Brodrick Daniel Coe, James Wilbur, James Stillings, Samuel Marsh, Enoch Morse, William Moore, Vandever Reed, Joseph Coberly, George Miller, John Gabriel, Dixon Mitchell, Samuel Ballinger, Alfred Hale, William Smith, Thomas Dodds, James Cochren, William Hoff, William Rowland, Benjamin Vickers, Thomas Dun, Elijah Kinney, Elijah Burroughs, L. Hibbard and John G. Hibbert.