In the impregnation process, when pressure is applied, the dry fiber preform is first compressed and then unloads as the matrix flows within the pores. It has been shown earlier that for glass mat-reinforced thermoplastic (GMT) materials, the time to provide complete infiltration of the matrix is significantly shorter than the unloading time of the reinforcement. This effect is exploited here to control the processing time in order to provide full infiltration but limited relaxation of the fiber bed, thereby producing a graded fiber content structure. Symmetric glass fiber/polypropylene (PP) composites were impregnated for different processing times to produce parts with a higher fiber content on the surface, decreasing towards the center. The elastic modulus in bending of the GMT parts was measured by a three-point flexure test while the distribution of fibers in the matrix was quantified using optical microscopy combined with image analysis. The flexural modulus of GMT was found to decrease with the impregnation time, in good agreement with the prediction from infiltration and mechanics theory. Controlled processing could therefore be used to maximize the bending stiffness of GMT.