In this work, to improve the interface interaction between natural fibers and a cementitious matrix, several Colombian Angustifolia kunth bamboo fibers were modified using different pretreatment methods before being covered with an inorganic phase of manganese oxide. The coating was performed by placing the fiber in contact with a manganese salt solution at low concentration, followed by drying of the fibers before performing physicochemical characterization. The fibers were characterized by atomic absorption, X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The coated fibers were added to a cementitious matrix, and the mechanical properties of the resulting composites were measured to determine the optimal treatment for reinforcing the cementitious matrix. The manganese content in the modified natural fibers obtained by atomic absorption was between 0.7 and 1.23 wt %, and the manganese oxide was deposited in a uniform manner onto the surface of the fiber, as indicated by the SEM images. The XRD patterns indicate some variation of the crystalline structure of cellulose, the TGA analysis indicates that the raw fiber lost more weight than did the covered fibers, and the FTIR revealed the vibrational bands associated with manganese oxide vibration. The mechanical properties measured by monitoring the flexural strength indicate the interplay between the physicochemical characterizations and the mechanical performance. Various samples of the composite material were prepared by varying the length, and the percentage of the addition of the fiber, and the flexural strength, toughness, energy index, and total energy of the samples were measured in all fibrocement materials; the measurement results indicate an improvement in the flexural strength of the materials occurred for the composite material synthesized with the addition of 2% fibers of 2.5 cm length.