To shed crucial mechanistic insight into these new degradation processes, we present the first application of real-time time-dependent density functional theory (RT-TDDFT) for understanding photo-induced degradation mechanisms in PFAS. While DFT calculations have become more common in environmental research, prior DFT studies on PFAS have only focused on reactions on the electronic ground-state potential energy surface. In contrast, the RT-TDDFT formalism used in this work can describe electronic excited-state dynamics beyond conventional ground-state DFT to explore photo-induced mechanisms and bring a fundamental understanding of these degradation processes. In other words, the photo-induced degradation mechanisms in PFAS inherently occur on electronic-excited-state potential surfaces that cannot be treated with ground-state DFT, and excited-state RT-TDDFT is required for capturing the resulting photochemical reaction dynamics.