The development of low-cost hydrogen evolution reaction (HER) catalysts that can be readily integrated into electrolyzers is critical if H-2 from renewable electricity-powered electrolysis is to compete cost effectively with steam reforming. Herein, we report three distinct earth-abundant Mo-based catalysts, namely those based on MoSx, [Mo3S13](2-) nanoclusters, and sulfur-doped Mo phosphide (MoP|S), loaded onto carbon supports. The catalysts were synthesized through facile impregnation-sulfidization routes specifically designed for catalyst-device compatibility. Fundamental electrochemical studies demonstrate the excellent HER activity and stability of the Mo-sulfide based catalysts in an acidic environment, and the resulting polymer electrolyte membrane (PEM) electrolyzers that integrate these catalysts exhibit high efficiency and durability. This work is an important step towards the goal of replacing Pt with earth-abundant catalysts for the HER in commercial PEM electrolyzers.