AbstractFiber-reinforced polymer composites are used as emerging materials to repair oil and gas transmission pipelines without stopping the flow of fluids inside. These pipelines are subjected to harsh environment conditions such as being buried under soil and being exposed to high humidity, high temperatures, and different chemicals. It is important to predict the useful life of composite repairs in these harsh conditions in order to design better and to have robust repair solutions. This study evaluates mechanical properties, namely tensile strength tensile modulus, flexural strength, flexural modulus, and lap shear strength, of composite laminates when exposed to harsh environmental conditions such as hot-wet [70°C/85% relative humidity (RH)] for 0, 2, 4, and 6 weeks, steam autoclave [121°C and 1,05,000 Pa (1.05 bar)] for 0, 6, and 12 h, and ultraviolet (UV) ray exposure of type UVA-340 at 50°C for 0 and 2 weeks. The glass fiber–reinforced polymer (GFRP) composite laminates are prepared by hand layup process and are cut using a water-jet cutting process to avoid machining defects and to keep a good surface finish. The specimens are exposed to these corresponding environmental conditions for different periods of time, and the changes in their mechanical properties are studied. The tensile strength, flexural modulus, and lap shear strength values are compared with unexposed specimens. Decreases in strength and modulus values are observed for exposed specimens. A simple empirical expression is developed to predict the usable lifetime of a composite repair system exposed to such different harsh environmental conditions.