This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group Pi, a = 6.868 angstrom, b = 6.921 angstrom, c = 6.880 angstrom, alpha = 93.000, beta = 94.400, gamma = 96.220). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg2+ ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg2+ ions in Mg2Mo6Se8.