Heterogenous electrocatalysis is a surface process often involving electron transfer between active catalyst and reactants to the products. MoS₂ as a heterogeneous electrocatalyst for CO₂ reduction (CO₂RR) is widely studied and serve as a prototypical model for transition metal dichalcogenides materials. Knowledge of chemical and electronic states is crucial for optimization of the CO₂RR reaction and modern electronic-structure approaches have been employed here to great effect. Less appreciated has been the role of spectroscopy in extracting mechanistic information, particularly in catalytic processes. With atomic-level resolution and environmental sensitivity, X-ray adsorption spectroscopy (XAS), has long had to potential to access such information, however the inherent complexity of these measurements has meant that computational approaches are required. In this work, we advance an approach for operando simulation of the Sulfur K-edge in MoS₂ as it proceeded through different states during a CO₂RR reaction. By employing the effective screening medium method and a multi-determinant XAS approach, we reveal a new mechanism of the CO₂RR on MoS2, where the initial low biases affected the formation of S defects and low energy reaction sites, while higher voltages provided the surface electron density required to overcome the reaction barrier and stabilize the transition state. Each of these regime is accompanied by distinct spectral line shapes and intensities, which are clearly resolved in the experiments. We anticipate that this work will pave the way toward using in operando XAS to improve fundamental understanding of electrocatalysis in general.