On January 11, 1815, Allen County was officially formed using land from Warren and Barren Counties. It was named for Lt. Colonel John Allen, who raised one of the first regiments in the state. He was killed by Indians in route to Detroit during the Battle of the River Raisin.
A courthouse fire in 1902 resulted in the loss of some county records.
Warren, Barren, Monroe and Simpson in addition to Macon and Sumner counties of Tennessee.