Here we report a novel one‐pot approach to synthesize MoCx catalysts at much lowered temperature for hydrogen evolution reaction (HER) in acidic solution. A room‐temperature co‐precipitation reaction among Ni2+, MoO42− and 2‐methylimidazole in an aqueous solution produced a Mo−Ni−imidazole complex, which was used as the precursor to fabricate MoCx. The morphological and chemical properties of the obtained MoCx affected by calcination and other additive metal sources (like Fe, Co, Cu) were investigated in detail. We demonstrated that Ni−Mo alloying took place prior to MoCx formation during heating, which catalyzed the crystallization of MoCx at relatively low temperature. The MoCx/Ni‐650 catalyst exhibited a quite low overpotential of 172 mV at 20 mA cm−2. The high performance originates from the synergistic effect between MoCx and Ni−Mo alloy, high ratio of Mo3+/Mo2+, as well as high electrochemically active surface area and low charge‐transfer resistance. This work would open a new way to the design and fabrication of other transition‐metal carbide catalysts.