Miss. Code § 21-31-13
The provisions of Sections 21-31-1 through 21-31-27 shall include all full paid employees of the fire and/or police departments of each municipality coming within its purview, including the chiefs of those departments. All appointments to and promotions in said departments shall be made solely on merit, efficiency, and fitness, which may be ascertained by open competitive examination and impartial investigation. No person shall be reinstated in, or transferred, suspended, or discharged from any place, position or employment contrary to the provisions of Sections 21-31-1 through 21-31-27. The governing authorities of the municipality may, with the approval of the civil service commission, extend the benefits of Sections 21-31-1 through 21-31-27 to other full time employees of the municipality.
All incumbents and future appointees shall be subject to civil service, except, however, those appointees now and hereafter serving as extra members.
Miss. Code § 21-31-13