Mo. Rev. Stat. § 51.050: Qualifications.

No person shall be elected or appointed clerk of the county commission unless such person be a citizen of the United States, over the age of twenty-one years, and shall have resided within the state one whole year, and within the county for which the person is elected one year just prior to such person's election; and every clerk shall after the election continue to reside within the county for which such person is clerk.