Until its closing in 1978, it was the oldest continuously used County Jail in Missouri. The original structure was completed in 1848 for $6,091.50. Slaves quarried the 2½ foot thick limestone blocks which they used for construction of the two-story building. Each story had one large room, reminiscent of old dungeons, equipped with 1 ¼ inch round rings bolted into the outer walls. The prisoners were shackled to these rings at the feet. The large room on the ground level, known as the “bullpen” held slaves destined for the auction block on Main Street.In 1871, the appearance of the second-story room was changed with the addition of iron box cells. Brought to Boonville by steamboat from St. Louis, these were installed using the occupants of the jail as laborers. The jail was to receive no additional major changes for another century.