Environmentally friendly FRP composites, made of natural fibers and bio-based polymer matrices, may be used as externally bonded reinforcement for civil structures or buildings subjected to moderate outdoor conditions, in replacement of traditional carbon/epoxy systems. However, a major drawback of natural fibers is their sensitivity to moisture which can affect both the mechanical properties of FRP composites and their adhesive bond with concrete. This research aims at studying the influence of hygrothermal ageing on the performances of “green composites” manufactured by hand lay-up process using unidirectional flax fabrics and a bio-based epoxy matrix. The test program consists in subjecting FRP laminates and FRP strengthened concrete slabs to accelerated ageing conditions under various combinations of temperature and humidity, and to natural ageing in outdoor environment as well. Aged laminates are then periodically characterized by tensile tests and interlaminar shear tests, while the bond properties of concrete/composite assemblies are assessed by pull-off tests. Results are discussed in the light of complementary physico-chemical investigations, in order to relate observed performance evolutions to actual microstructural changes or damage processes taking place in the material.