Single-atom catalysts (SACs), known for their high atomic utilization efficiency, are highly attractive for electrochemical CO2 conversion. Nevertheless, it is struggling to use a single active site to overcome the linear scaling relationship among intermediates. Herein, an isolated diatomic Ni-Mn dual-sites catalyst was anchored on nitrogenated carbon, which exhibits remarkable electrocatalytic performance towards CO2 reduction. The catalyst achieves CO Faradaic efficiency (FECO) over 90 % within the potential range of −0.6 to −1.4 V vs. reversible hydrogen electrode (RHE), and a nearly 100 % FECO at a current density of 325 mA cm−2 in the flow cell. The Ni-Mn-NC also exhibits long-term stability, maintaining FECO above 96 % for over 14 h. The density functional theory (DFT) studies further reveal that the synergistic effect of adjacent Ni-Mn centers effectively reduces the reaction barriers for the formation of *COOH and thus accelerates the reduction of CO2.