A glass fiber-reinforced polymer-matrix pultruded composite was characterized by digital microscopy, image analysis, mechanical tests, and aging by water absorption and UV exposure. Both longitudinal and transverse samples were obtained from a composite plate and prepared for the several experiments. Mosaics of images obtained by Scanning Electron Microscopy (SEM), with simultaneous high-resolution and wide spatial coverage, were automatically built and quantitatively analyzed. An image processing and analysis method was developed to obtain, in automatic fashion, the area fractions of fibers, filler and matrix, as well as the distribution of fiber sizes and shapes. The obtained microstructural measurements were used to estimate mechanical properties that were compared to those directly obtained from bending tests, with excellent agreement. The material was through immersion in water and the change in mechanical properties was evaluated, indicating significant losses. The water absorption curve was experimentally obtained and was fitted with the Fick model. Exposure to UV did not affect the mechanical properties when the tests were performed in the fiber direction. However, when tests were performed in a direction normal to the fibers, there was a consistent reduction of the Young modulus as a function of exposure time.