Electrochemical nitrogen reduction reaction (NRR) under ambient conditions has attracted considerable scientific and engineering interest as a green alternative route for NH3 production. Molybdenum is a promising candidate as an electrocatalyst for NRR as it has a suitable binding strength with N species. However, the design of an efficient Mo‐based catalyst remains elusive. To enhance the selectivity of NRR toward NH3, we have developed a carbon nanofiber catalyst embedded with molybdenum and cobalt (Co−Mo−CNF). Co with a strong ability to dissociate water enhances local proton source near Mo, where the hydrogenation step of the NRR occurs. A NH3 formation rate of 72.72 μg h−1 mg−1 and a Faradaic efficiency of 34.5 % were obtained at −0.5 V vs. RHE. We also attempted to provide a mechanistic understanding of the NRR via in situ attenuated total reflectance surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) and isotopic labeling experiments using 15N2 and D2O. The appropriate amount of water adsorbed on the surface of Co−Mo co‐embedded carbon nanofibers facilitated the formation of N−H bonds and promoted the protonation step of nitrogen reduction via proton source transfer from the water dissociation, which was confirmed by the results of in situ surface‐enhanced infrared absorption spectroscopy experiments and isotopic labeling experiments using D2O and 15N2.