In 1876, the county commissioners specified the following for modernizing the one story 1858 jail that had become to small: a building 20 x 36 feet, two stories high, to be built of stone and brick. The contract was awarded to F. D. Adams, a Mechanicsville builder, who completed it in October 1876. The first floor was used as living quarters for the jailer's family while the second floor cells housed inmates grouped by gender and race. The building served as the county jail until 1942 when a new jail was constructed behind the courthouse. The St. Mary's County Historical Society maintains the Old Jail as a museum. An original cannon from the "Ark", one of the ships that carried some of the first colonists to Maryland in 1634, is mounted in front of the Old Jail Museum.