Polyamide 6 (PA 6) components are exposed to coupled environmental loading conditions such as humidity and ultraviolet (UV) radiation, leading to pronounced changes in stiffness, viscoelastic dissipation, and long-term mechanical performance. Reliable constitutive descriptions of such materials therefore require a clear separation and quantification of moisture-induced plasticization and UV-driven aging mechanisms. This study provides an experimental basis for the investigation and interpretation of these effects within a thermoviscoelastic framework. The effect of hydrothermal conditioning is investigated for distinct moisture states ranging from dry-as-molded to fully water-saturated conditions. Dynamic mechanical analysis (DMA) employing relaxation tests and temperature-frequency sweeps under controlled and non-controlled humidity enables the separation of intrinsic moisture-dependent softening from humidity-induced testing effects and reveals moisture-driven shifts of the relaxation spectrum. UV-driven aging is studied on moisture-equilibrated PA 6 subjected to controlled short- and long-term irradiation. Gel permeation chromatography (GPC), differential scanning calorimetry (DSC), and DMA are combined to link molecular-scale degradation, thermal state, and macroscopic thermoviscoelastic response. Initial UV exposure results in stiffness increase with minor changes in molar mass and crystallinity, while prolonged irradiation leads to progressive chain scission, stiffness reduction, and increased viscoelastic dissipation. Master curve construction demonstrates alterations of the relaxation spectrum and a reduced applicability of time–temperature superposition. The results establish a quantitative experimental basis for the formulation and calibration of constitutive models for petro-based polymers under environmental aging.