By 1839 the Mormon utopian city of Nauvoo, located on the Mississippi River, was created, settled, and flourished. In 1844 the Mormon leader Joseph Smith was killed in the Carthage, Illinois jail. After close to six years of rapid development the Mormon city of Nauvoo, which rivaled Chicago as Illinois' largest city, saw a rapid decline. In 1846 the Mormons had left Illinois for the West in a mass exodus.

The state has a varied history in relation to Slavery and the treatment of African-Americans in general. Some slave labor was used before it became a territory, but Slavery was banned by the time Illinois became a state in 1818. The Southern part of the state, known as "Little Egypt", was largely settled by immigrants from the South, and the section was sympathetic to the South and slave labor. For a while the section continued to allow some slave labor on a migratory basis, but citizens were opposed to allowing Blacks as permanent residents. In the Illinois Constitution of 1848, reacting to such concerns, a provision was made for exclusionary laws to be passed. In 1853 John A. Logan, later a Union General in the American Civil War, introduced such bills and laws were passed to prohibit all African-Americans, including Freedmen, from settling in the state.

Chicago gained prominence as a Great Lakes port and then as an Illinois and Michigan Canal port after 1848, and as a rail hub soon afterward. By 1857, Chicago was Illinois' largest city.

With the tremendous growth of mines and factories in Illinois in the 19th century, Illinois played an important role in the formation of labor unions in the United States. The Pullman Strike and Haymarket Riot in particular greatly influenced the development of the American labor movement.

Further information: History of Chicago

Main article: Illinois in the Civil War

During the American Civil War, over 250,000 Illinois men served in the Union Army, more than any other northern state except New York, Pennsylvania, and Ohio. Beginning with President Lincoln's first call for troops and continuing throughout the war, Illinois mustered 150 infantry regiments, which were numbered from the 7th to the 156th regiments. Seventeen cavalry regiments were also gathered, as well as two light artillery regiments.

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