The global shift toward clean energy demands the development of efficient hydrogen storage ma-terials and hydrogen evolution reaction (HER) catalysts. MXenes—a family of two-dimensional transition metal carbides and nitrides—have emerged as promising candidates due to their high con- ductivity, chemicaltunability, andstructuralversatility. However, theatomic-levelfactorsgoverning their performance in hydrogen-related applications remain insufficiently understood. This work em- ploys density functional theory (DFT) to systematically investigate how compositional tuning and undercoordination affect hydrogen adsorption and catalytic activity across a diverse set of MXene structures. The study examines both M2C and M3C2 MXenes (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W), considering both pure and mixed-metal systems. The thermodynamic stability and hydrogen adsorption behavior of oxygen-terminated surfaces are analyzed, along with trends in surface func- tionalization stability across compositions. Under-coordination effects are explored through models of MXene edges and nanoparticles, focusing on zigzag 010 and armchair 110 terminations. These undercoordinated environments are shown to significantly influence hydrogen adsorption, with edge sites often exhibiting stronger binding than basal planes. Size- and morphology-dependent effects are also captured through nanoparticle models. Building on these insights, HER activity is eval- uated by calculating hydrogen adsorption free energies (∆GH ) and constructing volcano plots for representative systems, including Mo2TiC2O2 and related double-transition-metal MXenes. Over- all, this work demonstrates that both composition and undercoordination serve as critical design levers for tailoring MXenes toward hydrogen storage and HER applications. By establishing clear structure–property relationships, these findings offer a foundation for the rational design of MXene- based, earth-abundant alternatives to platinum catalysts and contribute to broader efforts toward sustainable hydrogen energy technologies.