Voronoi analysis is implemented to assess the influence of fiber content on the microstructure and mechanical properties of bulk-molding compounds containing different weight fractions of E-glass fibers (EGF) (5–20 wt%). The fiber distribution in the polymer matrix is analyzed by scanning electron microscopy followed by the Voronoi tessellations, radial distribution function and statistical calculations. The experimental results are compared to modelled microstructures. The derived microstructural descriptors allow us to correlate the fiber weight content and the degree of fiber distribution homogeneity with the mechanical properties of EGF-reinforced composites. The distribution of fibers in composites with 10 and 15 wt% of fibers could be considered as the most homogeneous. This is in a good agreement with the results of the flexural strength and dynamic mechanical analyses, which confirmed that the latter samples exhibit the highest level of reinforcement.