The sluggish reaction kinetics of the alkaline hydrogen evolution reaction (HER) remains to be overcome for the commercialization of water-alkali electrolyzers [1, 2]. In this work, we demonstrate that the metastable face-centered-cubic (fcc) α-MoC_1−x phase shows superior water dissociation capability and alkaline HER activity than thermodynamically stable, hexagonal-close-packed β-Mo₂C phase. In order to design high surface area fcc α-MoC_1−x, an ordered mesoporous α-MoC_1−x (MMC) was synthesized by a nanocasting method. In the MMC structure, the α-MoC_1−x phase facilitates the water dissociation reaction, while the mesoporous structure enables high dispersion of metal nanoparticles (NPs) and efficient mass transport. As a result, Pt NPs supported on the MMC (Pt/MMC) catalyst showed substantially enhanced alkaline HER activity in terms of overpotentials, Tafel slopes, mass and specific activities, and exchange current densities, compared to those of commercial Pt/C and Pt NPs supported on particulate α-MoC_1−x or β-Mo₂C. In particular, the Pt/MMC shows a very low Tafel slope of 30 mV dec⁻¹, which is the lowest value among the reported Pt-based alkaline HER catalysts, suggesting the critical role of MMC in enhancing the HER kinetics. Furthermore, Pt/MMC exhibited almost no current decay for after potential cycling test whereas Pt/C underwent a negative shift of polarization curve. The promoting effect of MMC support in the alkaline HER was further demonstrated with an Ir/MMC catalyst.