<p>Uranium mononitride (UN) is a promising advanced nuclear fuel featuring high inherent uranium density and excellent thermal conductivity. Although density functional theory (DFT) has been extensively used to explore the fundamental properties of UN, systematic and comprehensive studies on the thermomechanical performances of irradiation-defective UN remain limited. Formation energies of typical point defects in UN are strongly dependent on local chemical environments, with Schottky defects exhibiting distinct energetic characteristics under U-rich, N-rich, and stoichiometric conditions. The introduction of these defects systematically reduces the elastic moduli of UN, while both Poisson’s ratio (<i>σ</i>) and Pugh’s ratio (<i>κ</i>) increase accordingly. Vickers hardness (<i>H</i>_V) gradually declines with defect incorporation, as verified by empirical formula-based fracture toughness (<i>K</i>_IC) assessments. The elastic anisotropy of defective UN is quantitatively evaluated via anisotropy factors and intuitively visualized using three-dimensional (3D) contour plots. This work further systematically calculates the melting point (<i>T</i>_m) and minimum thermal conductivity (<i>κ</i>_min) of defective UN. Thermodynamically, defective UN is less stable than pristine UN at low temperatures, yet becomes stabilized at high temperatures owing to its reduced Gibbs free energy (<i>g</i>). The linear thermal expansion coefficient (CTE, <i>α</i>_L) increases sharply with rising temperature and eventually plateaus at a stable value. This atomic-scale insight into radiation-induced defect mechanisms and their modulation effects on thermomechanical and thermal performances is essential for addressing key technical bottlenecks and advancing the development of next-generation nuclear reactor fuels.</p>