The present study explores the effect of the aspect ratio of carbon fiber on the mechanical, thermal, and impact properties of carbon fiber-reinforced acrylonitrile-butadiene-styrene (ABS) matrix composites, which were produced using: (1) chopped carbon fibers with different lengths by means of twin-screw extrusion followed by injection molding; and (2) long fiber thermoplastic (LFT) pellets with different lengths by means of LFT followed by injection molding. The results revealed that the carbon fiber/ABS composites made by using carbon fiber/ABS LFT pellets exhibited the tensile, flexural, dynamic mechanical thermal, heat deflection temperature, and impact properties higher than those by using conventionally extruded carbon fiber/ABS pellets. It is noted that 12-mm-long LFT pellets contributed most to improving the mechanical and thermal properties of carbon fiber/ABS composites, compared to 6- and 24-mm-long LFT pellets. Meanwhile, the Izod impact strength of the composites was highest in the case of 24-mm-long LFT pellets. The improvements in the mechanical and thermal properties of carbon fiber/ABS composites were explained by different length distribution and aspect ratio of carbon fibers existing in the composites produced via conventional twin-screw extrusion and LFT processes, respectively.