Developing effective catalysts based on earth abundant elements is critical for CO2 electroreduction. However, simultaneously achieving a high Faradaic efficiency (FE) and high current density of CO (j(CO)) remains a challenge. Herein, we prepare a Mn single-atom catalyst (SAC) with a Mn-N-3 site embedded in graphitic carbon nitride. The prepared catalyst exhibits a 98.8% CO FE with a j(CO) of 14.0 mA cm(-2) at a low overpotential of 0.44 V in aqueous electrolyte, outperforming all reported Mn SACs. Moreover, a higher j(CO) of 29.7 mA cm(-2) is obtained in an ionic liquid electrolyte at 0.62 V overpotential. In situ X-ray absorption spectra and density functional theory calculations demonstrate that the remarkable performance of the catalyst is attributed to the Mn-N-3 site, which facilitates the formation of the key intermediate COOH* through a lowered free energy barrier.