Many studies have focused on atomically dispersed metal–nitrogen–carbon (Me–N–C) catalysts owing to their unique chemistry and high catalytic activities. Me–N–C catalysts have active centers resembling metalloporphyrins; thus, being heterogeneous analogs of homogeneous catalysts, their catalytic characteristics can be described by organometallic principles. In this regard, the high electrochemical activity of Ni–N–C catalysts for carbon dioxide reduction reactions (CO2RRs) is particularly difficult to understand because Ni2+ is a d8 species with a chemically inert axial site for intermediate binding in a square-planar ligand field. To resolve such a conundrum, we investigated the effects of different coordination geometries and Ni spin states on CO2RR activities—both of which influence the chemical activity of the Ni center. We used the grand-canonical density functional theory (GC-DFT) and the occupation matrix control method to properly include a finite potential effect, and to control the oxidation state of the Ni center, respectively. We elucidated that the generation of Ni+ directly impacts the CO2RR activity by providing strong intermediate binding energies to the Ni center, and a defective coordination environment is essential for stabilizing the Ni+ oxidation state. Our present study identifying governing factors for the high catalytic activity of Ni–N–C catalysts provides a design principle to develop high-performing catalysts for CO2RR.