Mo. Rev. Stat. § 221.020

Except as otherwise provided in this section and sections 221.400 to 221.420, the sheriff of each county in this state shall have the custody, rule, keeping and charge of the jail within his county, and of all the prisoners in such jail, and may appoint a jailer under him, for whose conduct he shall be responsible. In any first class county without a charter form of government which contains all or part of a city with a population of at least three hundred thousand inhabitants, the sheriff and the county commission, by mutual agreement, may have the control of the county jail transferred to the county commission.