The thermomechanical and mechanical properties of calcite-filled polypropylene (PP) composites and their short glass fiber (SGF)-reinforced hybrid composites were studied herein. The interfacial adhesion strength and the influence of the combined particle filler and short fiber content were examined to characterize the positive hybrid effect. The thermal stabilities of the hybrid composites were analyzed by thermogravimetric/differential thermal analysis. Consequently, the increase in the calcite and SGF contents was caused by the increasing melting and decomposition temperatures. In contrast, the calcite and SGF contents had minor effect on the melting temperatures. In the mechanical tests, the samples were characterized by tensile, flexural, and unnotch impact properties. The presence of calcite and SGF in the composites increased the interfacial adhesion strength and enhanced the tensile and bending properties, but reduced the tensile elongation and the impact strength. The effectiveness of the interfacial adhesion strength was characterized by a dynamic mechanical analysis. The viscoelastic properties indicated that the fiber matrix interaction was enhanced by incorporation of calcite and SGF. The morphology of the fracture surfaces was investigated by scanning electron microscopy. These observations were proven by comparing the thermomechanical and mechanical test results aimed to identify the SGF/matrix interaction and the positive hybrid effect.