Abstract: Electrochemical synthesis of ammonia represents a green and environmentally friendly method distinct from traditional Harper-Bosch processes, which demand stringent conditions. However, identifying a catalyst with high selectivity and catalytic activity to cleave the robust triple bond of N₂ remains a formidable challenge. Herein, we present a systematic study on the geometrical and electronic structure, intensity of N₂ adsorption, reaction intermediates, change in Gibbs free energy, and desorption of by-product hydrazine for the nitrogen reduction reaction employing a MnN_x-graphene (x = 3, 4) catalyst from a theoretical perspective. The computational results reveal that MnN₃-graphene exhibits superior catalytic performance predominantly via the distal mechanism, with a low potential of 0.49 V. Moreover, the detachment of the produced NH₃ is facilitated with a free energy of only 0.27 eV, significantly lower than those of previous catalysts, ensuring the exceptional durability of MnN₃-graphene. This study offers theoretical insights guiding the exploration of single Mn atom catalysts in ammonia synthesis.