This study investigates the influence of nano silica on the mechanical performance and interfacial behavior of fiber-reinforced cementitious composites (FRCCs) incorporating steel, glass, polypropylene, and raffia fibers. The objective is to evaluate the impact of nano-silica content (0%, 1.5%, 2.5%, and 3.5%) on workability, compressive strength, fiber–matrix bond strength, and tensile response under uniaxial loading. The addition of nano-silica reduced flowability due to its high surface area and water demand but enhanced compressive strength, reaching a maximum value of 77.14 MPa at 3.5 percent nano-silica. Field Emission Scanning Electron Microscopy (FESEM) confirmed matrix densification and refinement of the interfacial transition zone (ITZ) in nano silica modified mixtures, supporting the observed strength gains. Single-fiber pull-out tests revealed that nano silica significantly improved average and equivalent interfacial bond strengths. Steel fibers exhibited the most consistent bond improvement, while raffia fibers demonstrated the weakest performance. Longer fiber embedment enhanced bond strength and energy absorption for high-modulus fibers. Equivalent bond strength trends indicated a strong dependency on fiber type and embedment length. Direct tensile tests demonstrated that nano-silica significantly enhanced the tensile strength and ductility of composites. The most substantial improvement was observed in steel fiber-reinforced composites, with cracking strength increasing by 166.7% and ductility by 143.3% at 2.5% NS. Correction factors were proposed to align theoretical tensile predictions with experimental results. Overall, nano silica proved highly effective in improving FRCCs by densifying the matrix, strengthening the fiber matrix interface, and enhancing mechanical performance under tensile loading.