Composite materials reinforced with fibers, particularly polymeric types, are widely used in various structural and material-oriented applications. However, during service, these composites are often exposed to varying climatic conditions cuduring service, necessitating improved durability and mechanical performance. This study investigates the impact of thermal-shock conditioning on the mechanical and viscoelastic properties of multi-walled carbon nanotube (MWCNT)-enhanced glass fiber reinforced polymeric (GFRP) composites. Samples were subjected to alternating extreme temperatures (+70 ℃ for 36 h and –60 ℃ for 36 h) to simulate harsh operating environments. Tensile tests revealed that composites with 0.1 wt.% MWCNT content exhibited the highest ultimate tensile strength (UTS) among all tested configurations, with thermally conditioned specimens showing a further enhancement in UTS compared to unconditioned ones. The improvement in tensile behavior is attributed to stress redistribution facilitated by residual stresses and differences in coefficients of thermal expansion during thermal-shock cycles. Dynamic Mechanical Thermal Analysis (DMTA) demonstrated increased stiffness and damping properties in MWCNT-reinforced composites, with minimal shifts in glass transition temperature (Tg). Scanning Electron Microscopy (SEM) analysis confirmed uniform dispersion of nanoparticles and effective stress transfer mechanisms within the matrix. These findings underscore the potential of thermally conditioned MWCNT-GFRP composites for high-performance applications in aerospace, automotive, and structural engineering sectors, where materials are exposed to extreme environmental conditions.