Developing bioactive implants with strong mechanical properties and biomineralization activity is critical in bone repair. In this work, modified cellulose nanofiber (mCNF)-reinforced bioactive glass (BG)-polycaprolactone (PCL) hybrids (mCNF–BP) with strong biomechanics and good apatite formation ability were reported. Incorporating mCNFs shortens the forming duration of the hybrid films and enhances the biomechanical performance and in vitro apatite-formation capability. The optimized biomechanical performance of the optimal hybrid materials is produced at a relatively high mCNF content (1.0 wt%), including a considerably higher modulus of elasticity (948.65 ± 74.06 MPa). In addition, the biomineralization activity of mCNF–BP hybrids is also tailored with the increase in the mCNF contents. The mCNF–BP with 1.5 wt% and 2.0 wt% mCNFs demonstrate the best biomineralization activity after immersing in simulated body fluid for 3 days. This study suggests that mCNFs are efficient bioactive additive to reinforce BG-based hybrids’ mechanical properties and biomineralization activity.