The electrochemical CO2 reduction reaction (CO2RR) is a promising approach of using renewable power sources such as wind and solar to convert CO2 into value-added products. However, conventional methods of synthesizing high-performance CO2RR catalysts usually produce wastes and are not environmentally friendly. Herein, we developed a sustainable catalyst synthesis method by using cheap, abundant cornstarch as the feedstock, and doping it with nickel (Ni) from a simulated metal-containing wastewater, before finally doping it with nitrogen (N) to create a highly efficient metal-nitrogen-carbon (M-N-C) catalyst that is dominant by single atomic Ni sites without the need for an acid wash post-treatment. The cornstarch-based catalyst demonstrated a high Faradaic efficiency (FE) of 92% for CO production with a CO current density of 11.6 mA/cm2 at −0.8 V versus RHE. At the same Ni content under the same testing conditions, a catalyst prepared via conventional wet impregnation only attained a CO current density of 9.3 mA/cm2, and a catalyst prepared using more expensive graphene oxide achieved a CO current density of 11.5 mA/cm2 but with a lower FE (CO) at 81%. Findings from this work provide insights into using low-cost sustainable biomaterials and non-waste producing methods to produce effective electrochemical CO2RR catalysts.