Mo. Rev. Stat. § 77.240
When any vacancy shall happen in the office of mayor, by death, resignation, removal from the city, removal from office, refusal to qualify or otherwise, nominations of a successor may be made by any member of the council and selected with the consent of a majority of the members of the council, except in counties of the first classification with a charter form of government which have a population of at least nine hundred thousand inhabitants. The council may adopt procedures to fill any such vacancy consistent with this section. In the case of a temporary absence of the mayor or disability to perform the duties of his or her office, the president pro tem of the council shall perform the duties of mayor until the mayor shall return or such disability be removed; and during the time the president pro tem of the council shall act as mayor, the president pro tem shall receive the same compensation that the mayor would be entitled to. In counties of the first classification with a charter form of government and which do not contain a city with a population of at least four hundred thousand, in case of vacancy other than a temporary absence or disability, the person exercising the office of mayor shall cause a new election to be held; provided, when a vacancy occurs within six months of a municipal election, no election shall be called to fill such vacancy.
§ 77.240, RSMo