Mo. Rev. Stat. § 79.120: Mayor may sit in board.

The mayor shall have a seat in and preside over the board of aldermen, but shall not vote on any question except in case of a tie, nor shall he preside or vote in cases when he is an interested party. He shall exercise a general supervision over all the officers and affairs of the city, and shall take care that the ordinances of the city, and the state laws relating to such city, are complied with.