Mo. Rev. Stat. § 82.490
In all cities in this state now or hereafter having six hundred thousand inhabitants or more, and in all cities in this state not within a county, but constituting both a political subdivision and a city in its corporate capacity, the city treasurer shall be commissioned by the mayor, and shall serve for the term or period in such commission fixed and thereafter until his successor is duly elected or appointed and qualified. In the event of a vacancy in the office of city treasurer, arising from any cause, such vacancy shall be filled by appointment by the mayor, and the city treasurer so appointed shall serve until the first Monday in January next following the date of the election of his successor, and thereafter until his successor is qualified.
§ 82.490, RSMo