Original central core of capitol was built in the Greek Revival style. It features a circular drum tower capped with a dome and cupola, and stands at 119 feet tall. Later Neoclassical wings added at sides and back. The 1851 central portion of Capitol was designed and built by George Nichols replacing 1847 Capitol, designed by Stephen D. Button that burned.

However, different sources credit different architects. The state's own website credits the design to Barachais Holt, and claims it was built on the foundation of the burned Button Capitol. Another source at the Library of Congress says Nichols, not Button, designed the burned capitol. That source credits Nimrod E. Benson and Justus Wyman as supervising the new capitol's construction. It does not list the building's architect, but it states that Nichols strongly influenced the design.

Additions include the 1885 east wing, W. T. Walker architect, the 1905-06, south wing, Frank Lockwood & Charles Follen McKim, architects, the 1911-12 north wing, Frank Lockwood architect, the 1989-92 east portico, architect unknown. Construction is underway (2002) on the west wing.