Fibre Metal Laminates (FMLs) are very interesting materials due to their light weight coupled with their high stiffness, high fatigue resistance, and high damage tolerance. However, the presence of the polymeric matrix in the composite layers and of polymeric adhesive at the metal/composite interface can constitute an Achille’s heel for this class of materials, especially when exposed to a hot environment or water. Therefore, in the present article, aluminium/carbon fibre FML specimens were produced, aged by considering different hydrothermal conditions, and then, subjected to mechanical testing. The End-Notched Flexure (ENF) test was considered for this activity. It was found that the first ageing stage, consisting of submersion in saltwater, was very detrimental to the specimens, while the second stage, composed of high and low temperature cycles, showed an increase in the maximum load, probably due to a post-curing effect of the resin during the higher temperatures of the ageing cycles and to the dissolution of salt crystals during the subsequently ageing stages in distilled water.