The boundaries of Henry County were defined by the Territorial Legislature of Wisconsin in 1836, but it was not legally established until 1837 by the Iowa Territorial Legislature.  Prior to establishment of Jefferson County on the west in 1839, all of that part of the Black Hawk Purchase situated in the present county of Jefferson, was attached to Henry County for judicial purposes.

The county was named for Henry Dodge, who was a general in the Black Hawk War and was governor of the Wisconsin Territory.

The first courthouse in the state of Iowa was built in Mount Pleasant in 1839.  A second courthouse, used from 1871 - 1914 was the remodeled “Hill Hall,” on the same block as the county jail.  When it was put up for sale in 1914 it only brought $247.50.  The present building was completed in 1914.  

The earliest roads followed the tops of ridges and Indian paths.  Later, they cut straight against the plain.  Many of these roads still retain their original locations as originally surveyed and laid out.

Henry County was the only county in Iowa to have two military roads, present U.S. route 218 was one of them.