Structure, microstructure and hydriding properties of mechanically alloyed 2Mg-Ni mixture were investigated. Two different nanocomposites were synthesized by mechanical alloying (MA) in a low-energy planetary mill, namely MN100 (100 h of milling) and MN200 (200 h of milling). The formation of nanocrystalline Mg2Ni was detected as a function of the milling time. An appropriate combination of MA plus annealing under mild conditions accomplishes the complete formation of Mg2Ni phase. The pressure-composition isotherms of the two samples reveal different hydrogen storage capacities and plateau slopes. In addition, the low temperature Mg2NiH4 (LT) formed by hydriding/cooling of MN100 h decomposes at 190 °C, whereas this hydride produced from MN200 first transforms to the high temperature Mg2NiH4 and then decomposes near 245 °C. The differences in the hydriding/dehydriding properties of MN100 and MN200 were associated with the microstructure and structure of the phases formed during MA followed by heating under argon/hydrogen.