Boonville's old county jail is a stout limestone block with iron-grilled windows and a heavy wooden gallows shed behind it. Built in 1848 and used until 1978, it is one of the better-preserved 19th-century county jails in the Midwest — the cellblock, the iron doors, the sheriff's quarters at the front, and the hanging yard are all there to walk through.
The Friends of Historic Boonville run the building as a museum, with the jail interior, period rooms, and rotating exhibits on Cooper County history. Open afternoons most days from spring through autumn; modest admission.