Pulaski County was a part of Alexander County. As a result of an act of Legislature, dated November 3, 1843, the county of Pulaski was formed.
Pulaski County began at a point on the Ohio River in Range line between 2 and 3 east, of the Third Principal Meridian, running north with and on said line to Cache River, down and with said river to Alexander County Line; thence north to the southeast corner of Union County. Embracing an area of 192 square miles, Pulaski County is one of the smallest counties in the state of Illinois.
Also resulting from the November, 1843 Legislative act, the first commissioners were appointed to the county; they were Henry Sowers, Thomas Lackey, Jr. and Thomas Howard.

The Commissioners first job was to locate the county seat. One of the guidelines was to obtain a land donation of no less than 20 acres. They chose Caledonia, located on the banks of the Ohio River which was near the current town of Olmsted. The Commissioners were to submit their report to Thomas Forker, Justice of Peace.
The land in Caledonia was donated by Colonel Justus Post and his wife Eliza G. Post, thus making this the first deed recording for Pulaski County.

A court house was erected on the donated lant and in those days, building a court house seemed to have been a great undertaking, as in this case, the county was authroized by an act of the Legislature in 1847 to borrow $600.00 to finish the court house of Pulaski County. The act further authorized the county to levy a tax to build a jail.
Caledonia held the county seat until 1861. On February 13, 1861 Legislature passed an act authorizing the removal of the county seat from Caledonia to Mound City, causing Caledonia to become another deserted town.
In November, 1879 the Court house in Mound City caught fire. Nearly all of the records dating to 1860 were lost, which resulted in imperfection of record keeping of early records. Mound City still holds the County seat for Pulaski County.