To attain thermoplastic polymer composites with enhanced thermal and mechanical properties as well as long‐term durability, in this study, polyurethane‐coated carbon fiber (CF) and electron beam (EB) irradiation are adopted as an effective reinforcing filler and efficient crosslinking process, respectively. For this purpose, polyamide 6 (PA)‐based composites with different CF contents of 1–10 wt% were fabricated through melt‐compounding and compression molding, and then irradiated with various EB doses of 50–200 kGy. The SEM and FT‐IR data reveal that CFs are well dispersed in the PA matrix with excellent interfacial adhesion via specific intermolecular interactions, which are even enhanced for the composites with crosslinked PA matrices after the EB irradiation. As the result, the thermal stability (initial decomposition temperature and residue at 800°C) and dynamic mechanical properties of PA/CF composites increased noticeably with increasing the CF content and EB irradiation dose. The initial storage modulus of 1.90 GPa for neat PA at 30°C was improved significantly to 2.94 GPa by 10 wt% CF addition and to 4.67 GPa by 200 kGy EB irradiation. In particular, the long‐term mechanical properties of PA/CF composites, which were evaluated using a stepped isothermal method based on the time–temperature superposition principle, were found to be highly enhanced by the synergistic effect of CF filler reinforcement, EB‐induced PA matrix crosslinking, and improved interfacial adhesion.