In 1819, through the Indian Treaty of St. Mary’s, the United States Government acquired a tract of land known as the New Purchase.
Soon after this, the John Fugit and Griffy Griffith families entered the region and became the first settlers of the county.
The area comprising Decatur County was set off from this tract in 1821 and named for Commodore Stephen Decatur, a naval hero.
In April of 1822, the court was established and government set up in May of that year. The site of the county seat was chosen in June 1822 and named Greensburg by the wife of its founder, Thomas Hendricks for her home in Pennsylvania.
The first county building was a jail built in 1823 on the southeast corner of the courtyard.