Mo. Rev. Stat. § 78.460
The council shall consist of five members, and all persons now eligible for the position of councilman under the laws governing cities of the third class shall be eligible to serve as councilmen under sections 78.430 to 78.640. The term of councilmen shall be three years, except that of the first council elected after the adoption of sections 78.430 to 78.640, one member shall serve for one year, two for two years, and two for three years. Those councilmen receiving the highest number of votes at the first municipal election shall serve for the three-year term, those receiving the next highest shall serve for the two-year term, and the other for the one-year term. In counties of the first class with a charter form of government which have a population of at least nine hundred thousand inhabitants should a vacancy occur in the office of councilman by death, resignation or otherwise, a special election may be called by the council for the purpose of filling the vacancy, and the person elected at such special election shall serve only for the unexpired term or until his successor is elected and qualified. In all other counties should a vacancy occur in the office of councilman by death, resignation or otherwise, the council shall select a successor who shall serve until the next regular election. All councilmen shall be elected at large. They shall qualify and their terms of office shall begin on the first Monday after their election. The terms of office of the mayor and councilmen or aldermen in such city, in office at the beginning of the terms of office of the council first elected under the provisions of sections 78.430 to 78.640, including all boards and commissions, shall cease and determine and the terms of office of all other city officers, whether elective or appointive, in force in such city except as herein provided shall cease and determine as soon as the council shall by resolution declare, except that the council may continue the board of public works, and the library, hospital and park boards for such time or times after organizing under sections 78.430 to 78.640 as the interests of the city in its judgment may require.
§ 78.460, RSMo