Electrochemical reduction of CO2 has the potential to reduce greenhouse gas emissions while providing energy storage and producing chemical feedstocks. A mechanistic understanding of the process is crucial to the discovery of efficient catalysts, and an atomistic description of the electrochemical interface is a major challenge due to its complexity. Here, we examine the CO2 → CO electrocatalytic pathway on Ag(111) using density functional theory (DFT) calculations and an explicit model of the electrochemical interface. We show that the electric field from solvated cations in the double layer significantly stabilizes key intermediates—*CO2 and *COOH. At the field-stabilized sites, the formation of *CO is rate-determining. We present a microkinetic model that incorporates field effects and electrochemical barriers from ab initio calculations. The computed polarization curves show reasonable agreement with experiment without fitting any parameters.