We focus on the chemical transformations involved in the 2NaNH2 + MgH2 system subjected to thermal and mechanical inputs. Transformations occurring on the powder mixture during thermochemical and mechanochemical processes are described by ex-situ X-ray powder diffraction (XRPD), FT-IR spectroscopy, differential scanning calorimetry and manometric measurements. In the thermally activated samples, the reaction take place through the fast formation of Mg(NH2)2 and NaH via metathesis reaction between NaNH2 and MgH2 at 125 °C. FT-IR analysis confirms the presence of unreacted NaNH2 and a new Na-amide phase that could be ascribable to tetramide, Na2Mg(NH2)4. At higher temperature, the formation of new imide-amide phase is detected, stables up to 300 °C. On the other hand when the initial mixture is subjected to mechanochemical processing for longer milling time (50 h), only Mg(NH2)2 and NaH are produced. The hydrogen desorption reaction of the as-milled Mg(NH2)2–NaH mixture starts at 100 °C together with the formation of the NaMg(NH2)(NH) imide-amide phase, equally to the initial mixture.