The mechanical, structural, and morphological properties of polystyrene (PS) under fatigue combined with accelerated aging were investigated. The results revealed that under the condition of fatigue (tension–tension) combined with UV-B exposure, in the periods of 336 and 575 h, the mechanical resistance presented is lower when compared to PS subjected to only fatigue. The modifications produced by the irradiation were evaluated by physical-chemical tests, which resulted in changes in the properties of the degraded materials, such as reduced molecular mass due to chain cleavage and increased crystallinity. The predominant failure mechanism in mechanically fractured specimens was also influenced by irradiation, so that the hardening of the surface layer was characterized by Vickers microhardness.