The present research examines the effects of UV radiation, moisture and elevated temperature on the mechanical properties of GFRP pultruded profiles. Flexural, compressive and tensile properties of different GFRP sections were studied after they were exposed for 1000, 1500, 2000 and 3000 h to UV radiation and water vapour condensation cycles. Mechanical tests, including three-point bending, compression and tension tests, SEM analyses, and statistical studies were conducted to gather comprehensive results. The results showed that the mechanical properties of various GFRP sections generally decreased with the duration of conditioning: however, the rate of the decrease that was only slight up to 1000 h, increased rapidly during 1000–2000 h, and again it was slow during 2000–3000 h. The maximum reductions were 34%, 28% and 23% after exposure to 3000 h cycles for bending, tensile and compression tests, respectively. In the bending tests, where inter-laminar shear failure controls ultimate strength, the degradation was greater compared to the situation where fibres fracture controls the failure. Further, regarding the cross-section parameter, it was concluded that the thickness and perimeter are the effective factors; the thinner the sample and the larger the perimeter, the greater is the reduction. However, the maximum effect of the cross section in terms of thickness and perimeter did not exceed 13% among all sections.