Electrochemical nitrogen (N₂) reduction reaction (ENRR) is a promising alternative to the traditional Haber–Bosch process for ammonia (NH₃) production, offering milder conditions with lower environmental impact. However, there are significant limitations to achieving efficient ENRR, including low Faradaic efficiency (FE) and limited NH₃ production rates, due to the slow inert N₂ activation and competing hydrogen evolution reaction (HER). This study introduces mesoporous tungsten oxynitride nanofiber (mWON_x NF) as highly efficient electrocatalysts for ENRR. Synthesized via electrospinning, calcination, and nitridation processes, the mWON_x NF features multivalence states and oxygen vacancies, enhancing N₂ activation by providing abundant active sites and facilitating electron transfer to the N₂ molecule. Moreover, the three-phase boundary (TPB), a key factor in enhancing ENRR, is facilitated by the one-dimensional mesoporous structure of the material, thereby increasing local N2 concentration near the catalyst surface while suppressing the HER. Consequently, the mWONx NF achieves a remarkable FE of 32 % at −0.2 V_RHE and an NH₃ production rate of 27 μg h⁻¹ cm⁻² at −0.5 V_RHE in 0.1 M HCl. This work not only demonstrates the viability of tungsten-based electrocatalysts for sustainable NH₃ production but also emphasizes the importance of TPB formation strategies in electrochemical applications involving poorly soluble gases.