To explore low-cost, high-efficiency, and noble-metal-free catalysts for electrocatalytic water splitting in both acidic and alkaline media, the metal-metal carbide Janus hierarchical structure comprising Mo and β-Mo2C embedded on a carbon layer (Mo/β-Mo2C)@C is synthesized by a hydrothermal reaction and subsequent low-temperature magnesium thermic process. Systematic characterization by XRD, XPS, Raman scattering, and SEM/TEM reveals the successful formation of metallic Mo and β-Mo2C nanoparticles. The synthesized (Mo/β-Mo2C)@C has a large specific surface area and boasts highly efficient hydrogen evolution reaction activity including low overpotentials of 152 mV and 171 mV at a current density of 10 mA cm-2 and small Tafel slopes of 51.7 and 63.5 mV dec-1 in acidic and alkaline media, respectively. In addition, the catalyst shows outstanding stability for 48 hours in both acidic and alkaline media. The excellent catalytic activity originates from more active sites and greater electron conductivity bestowed by the carbon layer, which also improves the long-term stability in both acidic and alkaline solutions.