We have designed a new synthesis method for the complex Mg2NiH4 hydride, based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. A well-crystallized sample was obtained in a piston-cylinder hydrostatic press at moderate pressures of 2 GPa and temperatures around 750 °C from mixtures of MgH2 and Ni enclosed in gold capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features. Mg2NiH4 hydride shows a monoclinic symmetry with space group C2/c (No. 15). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) have been carried out to determine the hydrogen desorption temperature, the hydrogen contents and the cyclability of the compound under a hydrogen pressure of 10 atm. These studies reveal two different desorption temperatures that correspond to the loss of the labile hydrogen H3 leading to a stable hexagonal structure (Mg2NiH3) followed by the complete hydrogen desorption to give Mg2Ni. The cyclability of the hydride was studied through 10 cycles at a hydrogen pressure of 10 atm; a slight loss of reversibility was observed due the progressive oxidation to MgO due to traces of oxygen adsorbed within the DSC chamber.