Environmental processing, such as thermal cycling and ultraviolet (UV) exposure contribute to mechanical properties degradation of polymer composites. This paper focused on assessing the mechanical properties of triaxial carbon/epoxy (CFRP) composite laminates used in fabricating light-weight composite bridges when exposed to both thermal cycling and ultraviolet radiation. In this study, five test series with a total of 39 specimens were performed to evaluate such exposure effects on tensile, compressive, and shear behavior of CFRP laminates. The experimental program was divided into five groups, including (i) Pre-exposure (baseline) test group tests; (ii) Unstressed thermal cyclic aging where unloaded specimens are exposed to fifty thermal cycles from 71 degrees C to -33 degrees C; (iii) Tension thermal cyclic test group where specimens are pre-tensioned to 30% of the baseline ultimate tensile strength and are exposed to fifty thermal cycles from 71 degrees C to -33 degrees C; (iv) Compression thermal cyclic test group where specimens are subjected to 30% of the baseline ultimate compression strength while exposed to fifty thermal cycles from 71 degrees C to -33 degrees C; (v) UV radiation test group where unstressed specimens are continuously exposed to 313 UVB ultraviolet radiation for 750 h. Experimental results indicated that thermal cycling effects on both tensile and compressive moduli are larger than its effect on tensile and compressive strengths of the triaxial composite laminates. In addition, the effect of thermal cyclic exposure on specimens loaded in tension has a larger impact on laminate tensile properties. It was also found that thermal cycling effects on shear strength are larger than its effect on the laminate shear modulus and that the effect of thermal cycling of pre-compressed laminates has a major influence on the CFRP composite laminate shear properties. Results also indicated that exposure to ultraviolet radiation has a large effect on both tensile strength and modulus, as well as the compressive strength of CFRP laminates. However, it has a negligible effect on compressive and shear moduli, as well as laminate shear strength.