<p>This work presents a first-principles investigation of the structural, electronic, thermoelectric, and optical properties of orthorhombic monolayer Mg₃MoN₄ with Pmn2₁ symmetry to evaluate its potential for sustainable energy applications. Its mechanical and energetic stability were verified through 2D structural optimization, and its robustness was attributed by mixed ionic-covalent bonding characteristics. Mo-d orbitals dominate in the conduction region, affecting electron mobility, while the overlapping peaks between N-p and Mg-p orbitals around the valence band indicate significant hybridization and covalent bonding. While charge density calculation indicated significant orbital interaction, electronic band structure analysis showed semiconducting nature with a suitable band gap (2.7&#xa0;eV) for visible-light absorption. A high figure of merit (ZT) over a wide range of temperatures was calculated from thermoelectric analyses, which showed a balanced Seebeck coefficient, electrical conductivity, and low lattice thermal conductivity. This indicated outstanding efficiency for thermal-to-electric conversion. Significant anisotropy and significant absorption in both the visible and ultraviolet ranges were observed using optical simulations, confirming its applicability for light-harvesting and optoelectronic devices. Overall, Mg₃MoN₄ multifunctional performance makes it an attractive choice for advanced thermoelectric technologies and integrated solar energy conversion.</p> Graphical abstract <p></p>