This study investigates the influence of surface-modified carbon fibers (CFs) on the structural and mechanical properties of acrylonitrile-butadiene-styrene (ABS)-based composites. A comprehensive approach employing Fourier Transform Infrared Spectroscopy (FTIR), contact angle measurement, and thermogravimetric analysis (TGA) characterized the CF surface chemistry, wettability, and thermal stability. Specimens were prepared via injection molding and 3D printing processes, enabling systematic evaluation of tensile, flexural, and impact properties. Combined with Scanning Electron Microscopy observations of composite fracture surfaces, the study elucidates how modification treatments influence fiber–matrix interface bonding and mechanical enhancement mechanisms. The results indicate that after resizing treatment with silane coupling agents, the surface activity of CF and its interfacial compatibility with ABS were significantly improved, leading to a marked enhancement in the composite material’s overall performance. At a CF content of 9.62 wt%, the ABS-S-CF2 system exhibited optimal mechanical properties: The tensile strength and flexural strength of the injection-molded specimens reached 58.41 MPa and 81.51 MPa, respectively, representing increases of approximately 41.6% and 29.1% compared to neat ABS. The tensile strength and flexural strength of the printed specimens also reached 49.37 MPa and 80.19 MPa, respectively. Microstructural analysis indicates that the sizing treatment improves the interfacial bonding between CF and neat ABS.