The capitol dome is covered in zinc to provide a silvery facade which does not weather. The interior of the dome features a plaster frieze painted to resemble bronze, which illustrates scenes from Illinois history, and stained glass windows (including a stained glass replica of the state seal in the oculus of the dome). The seal featured in the top of the dome is the seal used by Illinois prior to the American Civil War. It differs from the modern seal in that the phrase "State Sovereignty" is above the phrase "National Union." After the Civil War, the legislature voted to reverse these phrases as they professed that National Union was the more important of these two concepts.

With a total height of 361 feet, the Illinois capitol is the tallest non-skyscraper capitol, even exceeding the height of the U.S. Capitol. The only state capitols taller than it are the non-classical designs found in Florida, Louisiana and Nebraska,