Converting carbon dioxide (CO2) into valuable products is one of the most important processes for a sustainable society. Especially, the electrochemical CO2 reduction reaction (CO2RR) offers an effective means, but its reaction mechanism is not yet fully understood. Here, we demonstrate that concerted cation–electron transfer (CCET) is a key catalytic step in the CO2RR to carbon monoxide. The first-principles-based multiscale simulation identifies a single cation that coordinates a CO2− intermediate adsorbed on Ag electrode. The CCET is experimentally verified by a collapse of the CO2RR polarization curves upon correcting for the thermodynamic activity of the cation. As further confirmation, a kinetic study shows that the CO2RR obeys first-order kinetics on the local cation concentration at the electric double layer (estimated by measuring the electrode surface charge). Finally, this work unveils the fundamental origin of different CO2RR activity depending on the species of alkali metal cation, and further highlights the importance of ion-pairing tendency of the cations for electrochemical interface design to achieve high-performance CO2 electrolysis.