Understanding the long-term photochemical stability of lignin is critical for its reliable use in outdoor applications such as biocomposites, coatings, and adhesives. In this study, the structural evolution of kraft lignin was systematically investigated under prolonged ultraviolet (UV) irradiation for up to 90 days through comprehensive molecular and surface analyses. Lignin progressively darkened, with the L∗ value decreasing to 27.0 and ΔE∗ reaching 5.0, and underwent initial depolymerization followed by molecular re-condensation. The oxidation process resulted in a substantial loss of aliphatic and phenolic hydroxyl groups, along with the formation of carbonyl- and carboxyl-enriched structures. Surface oxygen enrichment led to increased polarity and hydrophilicity, as evidenced by a decrease in water contact angle from 77.4° to 49.0° and a rise in negative surface charge from −42.4 mV to −54.7 mV. These molecular and surface-level changes in lignin demonstrate how UV exposure and weathering conditions alter the interfacial characteristics and structural stability of lignin in polymer systems.