Massively converting nitrogen gas to ammonia is a key process in modern agricultural and industrial fields. The conventional Haber–Bosch process for NH₃ production has to be carried out under extreme conditions, leading to high energy consumption and huge emission of greenhouse gases. Electrochemical N₂ reduction is a promising way for NH₃ production due to its sustainable process and feasibility in an ambient environment. In this work, we screen the transition metals (TM), including 26 elements, supported on two-dimensional (2D) Nb₂CN₂ (TM-Nb₂CN₂) for their applications in the electrochemical reduction of N₂ (NRR) based on first-principles calculations. We show that most SACs can bind with Nb₂CN₂ strongly through a TM-3N configuration. We find that Mn-Nb₂CN₂ is a promising candidate for the N₂ reduction reaction (NRR), with a low overpotential of 0.51 V through the distal mechanism. Importantly, TM-Nb₂CN₂ presents high selectivity to NRR by blocking the hydrogen adsorption and preventing the hydrogen evolution reaction. Moreover, the scaling relationship and Bader charge analysis provide an insightful understanding of the mechanism for NRR on single-atom catalysts (SACs) anchored on 2D MXenes. Our findings may guide the design of novel substrates for SACs with effectively improved performance.