Polymer/phlogopite composites have been rarely investigated despite their potential applications. We examined the effects of phlogopite on the mechanical, thermal, and rheological properties of two amorphous polymers, namely, ductile polycarbonate (PC) and brittle polystyrene (PS). The mechanical and thermal properties of PS/phlogopite composites improved with increasing phlogopite content. The modulus of PC/phlogopite composites showed a greater increase as a function of phlogopite concentration probably due to the good interfacial attraction between the phlogopite and PC, compared with the other filler-embedded PC composites. The mechanical and thermal properties (except for the modulus of PC/phlogopite composites) decreased owing to the PC chain scission (reduction in molecular weight) during extrusion, which was caused by the phlogopite infiltration into the PC matrix. The phlogopite’s effects on the abovementioned polymers were compared with those on other fillers such as kaolin and silica.