Focus has shifted to natural, renewably sourced fibers and thermoplastic composites as replacements for synthetic fossil fuel-based fiber-reinforced composites (FRCs) for automotive, marine, wind, and other applications where lightweighting and corrosion resistance are desired. Natural fibers can be reinforced with suitable polymer matrix materials to enhance thermal, mechanical, and chemical properties, broadly categorized as Natural Fiber Reinforced Thermoplastic Composites (NFRTCs). Due to their hydrophilicity, NFRTPCs, such as Flax Fiber–Polypropylene (FF/PP), suffer from fiber swelling due to moisture absorption. Cyclic swelling of fibers causes additional cracking and debonding. Since FRCs are required to last several years without degradation, an investigation into NFRTPCs performance over long-term environmental exposure is critical. A comparative investigation of NFRTPCs with equivalent Glass Fiber–Polypropylene (GF/PP) was undertaken for this study. To quantify weathering effects on the thermal and chemical properties of NFRTPCs, an accelerated aging protocol consisting of cycles of ultraviolet (UV) light exposure, elevated temperature, and humidity was formulated. Properties were evaluated for as-received material, flat panel consolidation, and a contoured thermoformed component to account for thermoforming effects. Thermoforming and UV radiation affect the microstructure of semicrystalline thermoplastic, directly influencing physical performance. Characterization was performed with Thermogravimetric Analysis (TGA) to assess moisture absorption and volatility, and Differential Scanning Calorimetry (DSC) to evaluate effects on thermoplastic crystallinity. Through these evaluations, a quantitative correlation can be established between constitutive material properties, effects of processing, and achieved microstructure and properties.