Mo-based catalysts constitute a significant portion of active non-noble H2 evolution reaction (HER) catalysts, but the role of Mo in conferring this activity remains debated. In this study, we utilize electrochemical, physical, functional, and computational approaches on a range of Mo-based catalysts to derive a unified explanation for Mo HER catalyst function. Across all catalysts studied, the formation of Mo3+ at cathodic potentials close to the thermodynamic HER onset drives activity, and activity between catalysts is therefore heavily determined by the ease of reduction from Mo4+ to Mo3+. As such, surface oxidation is detrimental to HER activity as it makes Mo3+ formation less facile. This theory provides a cohesive explanation for the origin of activity in Mo-based HER catalysts, emphasizing the specific role of Mo atoms in forming hydrides and carrying out the HER from Mo3+, with implications for future Mo catalyst design.