Wisconsin Territory was organized in 1836 from land formerly belonging to Michigan Territory encompassing the area as follows: south and east by Missouri, Illinois and Lake Michigan; east and north by Michigan, Lake Superior, and the International Line; westerly by the White Earth River and the Missouri River. In 1838 Wisconsin Territory was contracted to approximately the area of the present state, and the remainder of the territory was organized as Iowa Territory. In 1846 Iowa was admitted as a state, with its present boundaries. The remainder of the territory, being the residue of the original Wisconsin Territory, was unattached until the creation of Minnesota Territory in 1849.

 

The following table summarizes the formation dates of Iowa Counties. Note that counties have a complex history that this table does not adequately represent. For more detailed information, see the individual county website or consult the Histories of Iowa Counties at the State Library of Iowa.