At the general election in the year 1996, and every four years thereafter, the voters of each of the districts shall elect a county commissioner, who shall hold his office for a term of four years and until his successor is duly elected and qualified; and at the general election in the year 1882, and every four years thereafter, the presiding commissioner of the county shall be elected by the voters of the county at large, who shall hold his office for the term of four years and until his successor is duly elected and qualified. Each commissioner shall be a resident of the county and each commissioner elected from a district shall be a resident of the district from which such commissioner was elected. Each commissioner elected under the provisions of this chapter shall enter upon the duties of his office on the first day of January immediately after his election.