The low‐temperature (<210 °C) crystallographic structure, electrical conductivity, and thermal stability of Mg2NiH4powders compacted under isostatic pressures of up to 10 kbar were studied. A comparison is made with the corresponding properties of the noncompressed material. It has been concluded that under stress‐free hydriding conditions performed below 210 °C, a two‐phase hydride mixture is formed. Each of the hydride particles consists of an inner core composed of an hydrogen‐deficient monoclinic phase coated by a layer of a stoichiometric orthorhombic phase. The monoclinic phase has a metalliclike electrical conductivity while the orthorhombic phase is insulating. High compaction pressures cause the transformation of the orthorhombic structure into the monoclinic one, thereby resulting in a pressure‐induced insulator‐to‐conductor transition. Reduced decomposition temperatures are obtained for the compressed hydrides. This reduction is attributed to kinetic factors rather than to a reduced thermodynamic stability.