Glass fiber-reinforced polymer (GFRP) composite with neat epoxy matrix and GFRP nanocomposite with epoxy–organoclay nanocomposite matrix were prepared. Their tensile properties degradation at elevated temperatures and in alkaline as well as freeze–thaw cycling environments was studied by performing the uniaxial tension test. Optical microscope in the transmission mode was employed to observe the failed specimens. Results show that the tensile properties of both materials degrade with increasing testing temperature or increasing immersion time in alkaline solution. Interestingly, the degradation rate is reduced, though insignificantly, when epoxy nanocomposite is used as the matrix material. This can be attributed to the improved thermo-mechanical property and barrier property of epoxy nanocomposite. When exposed to freeze–thaw cycles, the tensile properties of both materials degrade first and then level off. However, in this case, organoclay introduction has little effect on the degradation rate of GFRP tensile properties. The failure mechanisms of both materials in different environments were also discussed.