The Capitol was initially designed by Englishman Thomas Fuller, who also designed the Parliament buildings in Ottawa, Canada. However, the Capitol that Fuller envisioned was never completed. In 1876, Fuller was replaced by two prominent American architects, Leopold Eidlitz and Henry Hobson Richardson.

Working together to evaluate and modify Fuller's design, they transformed the Capitol from a good building to a great one. Spanning four decades, the finished Capitol took 32 years to build, from 1867 to 1899. The final cost was a staggering $25 million dollars. Today, that same structure would cost a half billion dollars to build.

With the change in architects, the exterior design became a "battle of styles", in which Italian Renaissance, Romanesque and French Renaissance were blended. Four hundred feet long and three hundred feet wide, the Capitol has five stories with a full basement and attic. It is constructed principally of gray granite and has walls over sixteen feet thick at the foundation.