Mo. Rev. Stat. § 115.017: Election commissioners, where.

There shall be a board of election commissioners:

(1) In each county which has or hereafter has over nine hundred thousand inhabitants;

(2) In each city not situated in a county;

(3) In each city which has over three hundred thousand inhabitants on January 1, 1978, and is situated in more than one county;

(4) In each county of the first classification containing any part of a city which has over three hundred thousand inhabitants; provided that the county commission of a county which becomes a county of the first classification after December 31, 1998, may, prior to such date, adopt an order retaining the county clerk as the election authority. The county may subsequently establish a board of election commissioners as provided in subdivision (5) of this section;

(5) In each county of the first class which elects to have such a board through procedures provided in section 115.019.