Failure analysis of tensile-shear tested bonded composite-based single lap joints (SLJs) that have been subjected to two different levels of cyclic environmental loading is provided. Each test joint has at least one composite adherend which is made of glass fiber-reinforced polymer (GFRP); the second adherend may be aluminum, magnesium, or GFRP composite, and structural epoxy adhesive is used to join two adherends together for creating test joints. Scanning electron microscope (SEM) and energy dispersive spectrometry (EDS) are utilized to investigate the root cause failure of fractured surfaces that gives an insight into the recently published data that showed a significant effect of the cyclic heat on the static load transfer capacity (LTC) of the same SLJs. The SEM and EDS inspections show that the failure mode shifts from interfacial adhesive failure (ADH) to fiber tear (FT) for the GFRP/GFRP joints that have been exposed to cyclic heat with and without high relative humidity as compared to that at ambient condition. Further, failure analysis and discussion are provided.