Electrochemical oxidation of ethylene (C₂H₄) to ethylene glycol (EG) offers a sustainable route for chemical production. However, designing catalysts at the atomic level and precisely elucidating the microscopic mechanisms of catalytic processes remain a formidable challenge. Here, we report a well-defined Ag₁₄S(4-CF₃PhS)₁₂(PPh₃)₈ cluster (Ag₁₄) and its electrocatalytic performance and mechanism for the conversion of C₂H₄ to EG. The Ag₁₄ supported on carbon nanotubes (Ag₁₄/CNTs) achieved high selectivity (> 96%), EG productivity (73.43 μmol·h^‒1), and turnover frequency (5.25 s^‒1) at 2.4 V (vs. reversible hydrogen electrode). In situ characterization and theoretical calculations indicate that the catalytic process involves electrochemical activation of water to generate Ag-oxygen species, followed by the oxidation of C₂H₄ to form the intermediate ethylene oxide (EO), and eventually the ring-opening of EO to EG. The superior performance of Ag₁₄/CNTs is attributed to the role of mixed ligands in stabilizing high-valent silver and elevating the d-band centre, thereby reducing the adsorption free energy of key intermediates and suppressing the competing oxygen evolution reaction. This work provides a foundation for developing nano-sized silver-based catalysts for the electrocatalytic conversion of C₂H₄ to EG.