The electrocatalytic CO2RR to produce value-added chemicals and fuels has been recognized as a promising means to reduce the reliance on fossil resources; it is, however, hindered due to the lack of high-performance electrocatalysts. The effectiveness of sculpturing metal/metal oxides (MMO) heterostructures to enhance electrocatalytic performance toward CO2RR has been well documented, nonetheless, the precise synergistic mechanism of MMO remains elusive. Herein, an in operando electrochemically synthesized Cr2O3–Ag heterostructure electrocatalyst (Cr2O3@Ag) is reported for efficient electrocatalytic reduction of CO2 to CO. The obtained Cr2O3@Ag can readily achieve a superb FECO of 99.6% at −0.8 V (vs RHE) with a high JCO of 19.0 mA cm−2. These studies also confirm that the operando synthesized Cr2O3@Ag possesses high operational stability. Notably, operando Raman spectroscopy studies reveal that the markedly enhanced performance is attributable to the synergistic Cr2O3–Ag heterostructure induced stabilization of CO2•−/*COOH intermediates. DFT calculations unveil that the metallic-Ag-catalyzed CO2 reduction to CO requires a 1.45 eV energy input to proceed, which is 0.93 eV higher than that of the MMO-structured Cr2O3@Ag. The exemplified approaches in this work would be adoptable for design and development of high-performance electrocatalysts for other important reactions.