His most significant works are the interior space and mechanical systems of the Ohio Statehouse. This massive government building was erected between 1839 and 1861, with Kelley serving as one of four principal architects between 1854 and 1858. When he came to the project, Kelley basically began from scratch, as his predecessors had taken all plans and working drawings away with them. Walls and some flooring were in place, and first steps in erecting iron trusses for the roof had begun. Undaunted by the great scope of work before him, he saw it as fitting challenge for his abilities, stating the project was "a rare field for the exercise of ingenuity and invention in the correction of mistakes, careless conclusions and want of foresight." Seeing the building had no provision for heating or ventilation, Kelley designed an innovative steam heating system that was highly effective, at a time when central heating was a rare and expensive luxury. He was responsible for the finishing touches on the exterior of the building, as well as the design and engineering of most of the interior spaces.