Carbon fibers (CFs) and yarns are modified by a novel approach to improve their interfacial interaction with cementitious matrices to efficiently enhance the bond and herewith mechanical performance of carbon fiber-reinforced, cement-based composites. Anodic oxidation in cement pore solution is performed by applying different voltages and treatment times. Both parameters contributed to production of a significant quantity of crystalline products, confirmed by scanning electron microscopy. X-ray diffraction (XRD) and energy dispersive x-ray analysis (EDX) confirmed the presence of oxygen-containing calcium species on the fiber surface, supporting strongly the presence of calcite. Moreover, the thermal stability of the modified CFs was studied by thermogravimetric analysis, indicating a temperature-stable coating but also a degradation of the CFs when higher voltages had been applied. Single-fiber tension tests documented the influence of various treatment times and voltages on the tensile strength of the CFs. Additionally, single-fiber pullout tests were performed to examine the bond behavior of a single fiber embedded in a cementitious matrix. The results proved an enhancement of the bonding between modified fibers and cement-based matrix.