In the present work, highly effective Ni-MnO binary nanocomposite catalysts were designed and synthesized using a one-pot method from Ni-Mn based bi-metal organic frameworks (MOFs). These nanocomposites were introduced into MgH₂ through ball milling as catalysts to enhance the hydrogen storage properties of MgH₂. Through varying the Ni/Mn ratio in the bimetal MOFs, it is found that the Ni₁Mn₁₋MOF derived catalyst showed the best promotion effect on MgH₂. The MgH₂–10 wt.% Ni₁Mn₁₋MOF derivative demonstrated favorable overall performance with the low desorption peak temperature (218.2 °C) with a saturated hydrogen capacity of 6.42 wt.% and rapid hydrogen release/uptake kinetics. It can still reabsorb about 1.15 wt.% H₂ within 30 min at a temperature as low as 50 °C. Both performance tests (DSC and TPD) and structural characterizations (XRD, HRTEM, etc.) revealed that the synergistic role of in situ formed Mg₆MnO₈ and Mg₂NiH₄/Mg₂Ni phases for improving the hydrogen sorption properties of MgH₂. Theoretical calculations reveal that Mg₆MnO₈ destabilizes metal-H bonds in MgH₂ and Mg₂NiH₄, leading to an enhanced “hydrogen pump” effect of Mg₂NiH₄ for MgH₂. This research provides a strategy to rational design and preparation of bimetal MOF derivatives for the development of advanced hydrogen storage materials.