Experimental identification of the mechanical properties of a selected glass-vinylester structural composite is developed, performed, and analysed taking into account accelerated environmental ageing and three operating temperatures (–20, 20, and 55°C) corresponding to the operating temperature range for composite footbridges in the Central Europe. The main constituents of the composite fabricated using infusion technology are a bidirectional balanced stitched E-glass fabric and a flame retardant, vinylester resin. After homogenization, the composite reinforced with one fabric forms a single lamina and is modeled as a linear elastic-brittle orthotropic material. Full sets of material constants were identified for the initial and aged composites at the selected operating temperatures. The accelerated environmental ageing of the composite was performed on 4-layer symmetric laminate platelets protected with a 300-mm-thick gelcoat layer, using an ageing chamber and a relevant ageing programme. A comparative analysis was carried out in order to determine the effects of operating temperature and accelerated environmental ageing on the material constants of the GFRP composite. It is found that the composite tested can be modeled as a linear elastic-brittle orthotropic material to the level of ~20% of its strength in each strength test. The impact of the accelerated environmental ageing and operating temperature in the range from –20 to 55°C on the elastic/strength/ultimate strain constants of the selected E-glass/vinylester composite can be significant and different for individual constants.