The extensive use of Acrylonitrile Butadiene Styrene (ABS) raises challenges due to its vulnerability to oxidative degradation, primarily impacting its mechanical behavior. Photodegradation appears to be the pivotal driver in this degradation process. The main aim of the present study is to provide a comprehensive approach to the underlying mechanisms governing the transition from ductile to brittle behavior in ABS after exposure to UV-induced photodegradation. This study thus focuses on the multiphase nature of ABS, wherein polybutadiene (PB) nodules are dispersed within a matrix of styrene-acrylonitrile. Dynamic mechanical tensile tests have been employed to highlight aged layer mechanical impact by blocking the dissipative response of the matrix and applying a homogenous stress field, while atomic force microscopy (AFM) has been used to characterize the local mechanical properties of ABS finely. The evolution of global mechanical properties has been correlated with changes in microstructural behavior. One of the main contributions of the present work is the correlation between the formation of brittle layers with the brittle state of aged ABS. Mechanical testing using dynamical tensile test revealed a critical aging time marking the transition from a ductile to a brittle state of the 450 µm thick samples. Chemical analysis using infrared spectroscopy revealed evolutions in the material composition, especially in the carbonyl, hydroxyl, and PB unsaturation groups. Distinct thin oxidized layers on each exposed surface were observed using optical microscopy. Finally, AFM analysis on the exposed surface shows the stiffening of PB nodules, indicating the formation of brittle thin layers due to oxidation causing an apparent fragile behavior of ABS.