If current production and waste management trends continue, roughly 12 billion tons of plastic waste are assumed to accumulate in natural environments and landfills by 2050. Once in the environment, plastics lead to potentially severe consequences for sea- and wildlife with inestimable long-term implications for the world’s ecosystems. Within the last years, a discussion on microplastic (MP) emerged not only on a scientific level but also in public media. The main source of MP is the stepwise fragmentation from macro- to microplastic. Nevertheless, knowledge about degradation mechanisms, changes in chemical composition, morphology, and residence times is still limited. In a long-term accelerated weathering study, we exposed polystyrene (PS) tensile bars and particles using simulated solar radiation and mechanical stress. A multitude of analytical methods monitored the degradation. We verified that degradation proceeds in two main stages. Stage I is dominated by photooxidation in a near-surface layer. During stage II, microcrack formation and particle rupturing accelerate the degradation. An increasing amount of small MP fragments with high proportions of carboxyl, peroxide, and keto groups are continuously released. The enhanced surface area for adsorbing pollutants and forming biofilms modifies the uptake behavior and interaction with organisms together with potential ecological risks. We expect the proposed two-stage model to be valid for predicting the abiotic degradation of other commodity plastics with a carbon−carbon backbone.