Energy storage material development is primarily dependent on their design and theoretical exploration. Using density functional theory is a good way to achieve this. To do that, MgNiX (Bi, Ge, Sb) are proposed, and their full characteristics are investigated using density functional theory. MgNiX (Bi, Ge, Sb) are investigated in terms of their structural, mechanical, thermodynamic, optical, electronic, and dynamic properties. The formation energies for MgNiX (Bi, Ge, Sb) are −0.224, −0.258 and −0.421 eV/atom, respectively, implying synthesisability and dynamic stability. The evolution of elastic constants of materials demonstrates that all materials satisfy the Born stability criterion, hence MgNiX (Bi, Ge, Sb) are mechanically stable. Several polycrystalline elastic constants are computed and evaluated. The electronic band structures show that the valence band and conduction band overlaps, indicating the metallic nature of MgNiX (Bi, Ge, Sb) materials which are calculated by using PBE-GGA, PBE+SOC and PBE+U methods. The phonon modes are found to be in positive frequencies that confirms dynamical stability of the materials. The materials' free energy, entropy, specific heat capacity, Debye and melting temperatures and Grüneisen parameters are also obtained and discussed. The maximum optical conductivity is determined in the ultraviolet region for MgNiBi and MgNiSb as 73 at about 5.1 eV and 80 at about 5.50 eV, respectively and it is determined as 62 at 0.65 eV in the infrared region for MgNiGe.