FRCM composites based on long continuous fibres embedded in inorganic mortar matrix has been increasingly used to strengthen existing structures. The mechanical behaviour of FRCMs and their effectiveness in strengthening applications are highly dependent on the mechanical properties of the fibres and the mortar matrices. Research has revealed that the temperature and humidity of the curing environment has a pivotal impact on the development of the mechanical properties of cementitious mortar. Therefore, in this paper, the influence of the curing environment under moist or dry conditions with different temperatures on the degree of hydration and tensile strength of cementitious mortar are investigated. Then, direct tensile tests are applied to 30 steel-FRCM coupons which are cured in different environments to obtain the corresponding full range of tensile stress-strain responses. The effects of the strain measurement system, coupon size, clamping system and curing environment are investigated. Finally, the experimental tensile stress-strain response curves of the FRCM coupons are validated with the analytical results derived from the three different approaches to draw conclusions.