A combination of ab initio calculations and experimental methods (high-resolution neutron and powder X-ray diffractions) was used to solve the crystal structure of Mg x Mo6S8 (x = 1 and 2). It was shown that at room temperature, the latter are similar to the crystal structure of classic Chevrel phases (CPs) such as Cu x Mo6S8:  space group R3̄, a r = 6.494 Å, α = 93.43° for MgMo6S8 and a r = 6.615 Å, α = 95.16° for Mg2Mo6S8. For x = 1, one Mg2+ cation per formula unit is distributed statistically between inner sites. For x = 2, the second Mg2+ cation per formula unit is located in the outer sites. Peculiarities of the electrochemical behavior of the CPs as electrode materials for Mg batteries were understood on the basis of the analysis of the interatomic distances. It was shown that the circular motion of the Mg2+ ions between the inner sites in MgMo6S8 is more favorable than their progressive diffusion in the bulk of the material, resulting in relatively slow diffusion and Mg trapping in this phase. In contrast, in Mg2Mo6S8, the repulsion between the Mg2+ ions located in the inner and outer sites facilitates their transport through the material bulk.