Abstract Polystyrene ( PS ) is a commodity plastic recalcitrant to chemical recycling or upcycling processes. Approaches aimed at deconstructing PS by photocatalytic means struggle to generate high‐energy species capable of cleaving the robust C−H and C−C bonds of PS . We show that 9‐mesityl‐10‐methylacridinium perchlorate ( MA ) is capable of upcycling various grades of PS substrates into up to 40 % benzoic acid ( BAc ), formic acid ( FA ) and small proportions of acetophenone ( ACP ), under visible light (456 nm) or through solar radiation. Time‐resolved emission and absorption spectroscopy evidence that a reaction with oxygen is the primary photochemical step in oxygen‐saturated solutions, accounting for 77 % of the photons absorbed vs. 1 % for the direct reaction with PS (0.303 M in repeating units). These results are in agreement with a mechanism in which MA ‐mediated photo upcycling of PS to BAc occurs through the abstraction of benzylic H atoms by reactive oxygen species generated by energy or electron transfer from the excited state of MA . Addition of triplet O 2 to these radicals, followed by intra‐ or inter‐molecular hydrogen atom transfer (HAT) generates C‐ or O‐centered radicals then undergoing β‐scission or hydroperoxide fragmentation. The formation of intermediate oligomers functionalized by terminal carbonyl groups is demonstrated by both infrared analysis and MALDI TOF mass spectrometry. These oligomers undergo further photoinduced conversion even in the absence of MA , as evidenced by size exclusion chromatography analysis of the irradiated samples.