Mo. Rev. Stat. § 79.100

When any vacancy shall happen in the office of mayor by death, resignation, removal from the city, removal from office, refusal to qualify, or from any other cause whatever, the acting president of the board of aldermen shall, for the time being, perform the duties of mayor, with all the rights, privileges, powers and jurisdiction of the mayor, until such vacancy be filled or such disability be removed; or, in case of temporary absence, until the mayor's return.