In this study, electrochemically generated free chlorine (EC/Cl₂) was activated by UV_275nm irradiation to degrade atrazine, a commonly used herbicide, investigating the degradation kinetics and mechanisms. The potential at a RuO₂/IrO₂–Ti anode was optimized as 1.5 V vs. Ag/AgCl through electrochemical measurement methods. The effects of atrazine initial concentration (C₀) on the UV-EC/Cl₂ and EC/Cl₂ processes were evaluated. The atrazine degradation efficiency under low C₀ conditions is surprisingly distinct from that under high C₀ conditions. Under low C₀ (1 μM) conditions, UV-EC/Cl₂ degraded 60% atrazine at 60 min with ˙OH as the dominant species, while EC/Cl₂ only removed 6% atrazine within 60 min. Under high C₀ (20 μM) conditions, similar atrazine degradation efficiencies (about 90% within 180 min) were observed both in UV-EC/Cl₂ and EC/Cl₂. 50 μM nitrobenzene was used with 20 μM atrazine to conduct competition experiments, which confirmed the significant role of Cl˙ in atrazine degradation in the UV-EC/Cl₂ and EC/Cl₂ systems. During atrazine degradation by UV-EC/Cl₂ and EC/Cl₂, transformation products were compared according to high-resolution mass spectra, based on which possible degradation pathways were proposed. Synthetic reverse osmosis concentrate (ROC) was used to mimic the ROC from industrial wastewater and roughly evaluate the performances of UV-EC/Cl₂ for the degradation of emerging contaminants, including atrazine, ibuprofen, carbamazepine, and diclofenac. Overall, EC/Cl₂ can be effective at optimized anodic potential for the degradation of contaminants that are at high C₀ and quickly react with Cl˙, while UV-EC/Cl₂ is able to generate ˙OH and reactive chlorine species to efficiently decompose organic contaminants at varied concentrations.