Mica-filled polypropylene (PP)-based GMT (PP-mica-GMT) was prepared by a double-belt press and its mechanical properties were tested. The effect of the Mica content on the mechanical properties of PP-mica-GMT was investigated. It was found that with a lower mica content in matrix the tensile and flexural properties were improved; however, at a high mica content level, the tensile and flexural properties decreased. With respect to the impact strength, there is a maximum value at an approximate 20 wt % mica content level. These results were attributed to the influence of mica on fiber-matrix adhesion. It is most likely that low mica content enhances fiber-matrix adhesion; however, high mica content is unfavorable to fiber-matrix adhesion. Maleic anhydride-grafted PP (MPP) was used to enhance the fiber-matrix adhesion at a higher mica loading level for PP-mica-GMT. With increasing MPP content in the matrix, the tensile properties were significantly improved, whereas the Izod impact strength decreased. In addition, with 5 wt % MPP and 40 wt % mica in the PP matrix formulation, the effect of the mica particle size and glass fiber content on the mechanical properties of PP-mica-GMT was investigated.