Magnesium is an attractive hydrogen storage candidate due to its high gravimetric and volumetric storage capacities (7.6 wt.% and 110 gH2/l, respectively). Unfortunately, its use as a storage material for hydrogen is hampered by the high stability of its hydride, its high dissociation temperature of 573–673 K and its slow reaction kinetics. In order to overcome those drawbacks, an important advancement toward controlling the enthalpy and desorption temperatures of nano-structured MgH2 thin films via stress/strain and size effects is presented in this paper, as the effect of the nano-structuring of the bulk added to a biaxial strain on the hydrogen storage properties has not been previously investigated. Our results show that the formation heat and decomposition temperature correlate with the thin film’s thickness and strain/stress effects. The instability created by decreasing the thickness of MgH2 thin films combined with the stress/strain effects induce a significant enhancement in the hydrogen storage properties of MgH2.