Abstract: Solid-solution magnesium-based alloys have garnered significant attention for hydrogen storage applications. However, their practical implementation has been limited by their stable thermodynamic properties and sluggish kinetics. Herein, we report a novel nanoengineering approach to simultaneously enhance the kinetic and thermodynamic properties of Mg-based solid-solution alloys. Using Mg(In) alloys as a model system, we demonstrate this positive size effect through a two-step fabrication process. First, the Mg_0.9In_0.1 alloy was synthesized via ball milling combined with absorption/desorption cycles. Subsequently, the alloy was subjected to high-pressure milling under a 4 MPa H₂ atmosphere with immiscible Mn at a controlled molar ratio, resulting in Mg(In) nanograins uniformly embedded within the Mn-composite matrix (denoted as (Mg_0.9In_0.1)_xMn_1−x). The (Mg_0.9In_0.1)_0.25Mn_0.75 nanocomposite, with an average grain size of ~61 nm, demonstrated superior hydrogen storage properties. Compared with pure MgH₂, this material exhibits much lower onset and peak temperatures for hydrogen release, at ~120 and ~240°C, respectively. Moreover, enhanced kinetic performance, with a significantly lower activation energy of ~78.34 kJ/mol, and improved cycling stability, with 97% retention after 50 cycles, are achieved due to the Mg(In) nanograins, which remain well-preserved even upon multiple cycles. This study highlights that the synergistic combination of solid-solution formation and nanoscale engineering can effectively modify the thermodynamic and kinetic properties of Mg-based hydrogen storage alloys, offering a promising approach for the development of high-performance magnesium-alloy hydrogen storage materials.