Fibre-reinforced polymers (FRP) are a composite system which may be able to meet the demands of both new and existing infrastructures. Due to its constituents’ environmental impact, natural alternatives are becoming increasingly popular; however, their longevity still needs to be assessed prior to its use in civil infrastructure. In this thesis, the long-term performance and durability of natural fibres (flax) and resins (epoxidized pine oil (EPO) and furfuryl alcohol (FA)) were investigated, using 729 tension coupons and 36 structural insulated panels (SIP). Four phenomena were studied: wet-dry cycling, freeze-thaw cycling, cold temperature testing and mechanical fatigue testing. Flax-FRP (FFRP) exposed to 12 wet-dry cycles experienced maximum reductions of 12% and 19% in tensile strength and modulus, respectively, due to delamination at the fibre-resin interface. FFRP exposed to 300 freeze-thaw cycles experienced maximum reductions of 8% and 10% in tensile strength and modulus, respectively, due to delamination at the fibre-resin interface and damage within the fibres. In both cases, conventional epoxy and EPO underwent hydrolysis, and exhibited a decreased and increased glass transition temperature, respectively. To mitigate damage, flax was treated to reduce moisture absorption and improve fibre-resin bond, although similar deterioration was observed post-conditioning. Conversely, conventional glass-FRP showed no deterioration. Furthermore, FFRP tested at cold temperatures resulted in a 20% increase in stiffness, specifically the slope of the second linear phase of its bi-linear stress-strain response. FFRP-skinned SIPs tested at cold temperatures showed a 28% increase in capacity under flexural load; however, there was no impact on the load-deflection response within serviceability limits. Finally, FA-based carbon-FRP (CFRP) was tested in tension-tension fatigue. Based on a 2,000,000 fatigue life, the ASTM model predicted an allowable stress amplitude of 59% and the Whitworth model predicted a modulus retention of 65%. Conventional epoxy-CFRP had a predicted modulus retention of 80%. Through this study, it is clear that FFRP with a higher FVF will be more resilient against cyclic environmental exposure than those with fibre treatment; and, its performance will improve in cold climates. Furthermore, FA-CFRP’s fatigue performance is worst than epoxy-CFRP; however, this trade-off may be beneficial for its chemical resistance.