The county seat of Monona was originally Ashton, but changed to Onawa in 1858. At Onawa, county officials were housed in a small building known as Mechanics' Hall.
Onawa was constantly being challenged for the honor of being the county seat. The citizens of Onawa raised $12,500 to be used for a new courthouse if it were to retain the county seat. In an election against East Mapleton, Onawa won and a $7,000 courthouse was erected.
The present courthouse was built in 1892 in Onawa. It is of typically late Victorian architecture. The design is similar to that of the courthouse in Adair County.