In recent years, even if Fiber Reinforced Polymer (FRP) composites have been widely used for strengthening of civil buildings, a new generation of materials has been studied and proposed for historical masonry construction. These buildings, mainly made of stone work, are common in many areas of Europe and Asia and recent earthquakes has been the cause of many catastrophic failures. The brittleness of unreinforced historic masonry can be considerably reduced using new retrofitting lighter-weight materials such FRP, even if limitations were evidenced due to material and mechanical compatibility with poor substrates. Thus, fibrous reinforcements were used as long fibres incorporated into a cement or lime matrix, which better match with the properties of ancient masonry. The use of low strength fibers such as glass and basalt, respect to carbon, in presence of an alkaline matrix brought out durability issues, due to the chemical vulnerability of common glass and basalt fibres. The objective of this research is to explore the effects of selected aqueous environments and fatigue loading on the mechanical and physical properties of composite grids, made of E-CR (Electrical/Chemical Resistance) glass fibers and epoxy-vinylester resin, used as tensile reinforcement in new composite reinforced mortar systems. Glass-fiber-reinforced polymer (GFRP) coupons were subjected to tensile testing and a severe protocol of durability tests, including alkaline environment and fatigue tensile loads. Accelerated ageing tests were used to simulate long-term degradation in terms of chemical attack and consequent reduction of tensile strength. The ageing protocol consisted of immersion at 40 °C in alkaline bath made by deionized water and Ca(OH)2, 0.16% in weight, solution for 30 days. GFRP specimens aged and unaged were also tested under tensile fatigue cycles up to 1,000,000 cycles and a nominal frequency of 7.5 Hz. After this severe conditioning the tests indicate a good tensile strength retention of the GFRP in absence of fatigue loads, while a significant loss in fatigue life was experienced when both alkaline exposure and fatigue loads were applied.