Treating severe bone deformities and abnormalities continues to be a major clinical hurdle, necessitating the adoption of suitable materials that can actively stimulate bone regeneration. Magnesium phosphate (MP) is a material that has the ability to stimulate the growth of bones. The current study involved the synthesis of mesoporous MP and lanthanum (La)-doped nanopowders using a chemical precipitation approach. The nanopowders were analyzed using several techniques, including XRD, FTIR, HR-TEM, BET, XPS, and FE-SEM. The results confirmed the nanopowders' size of less than 40 nm and the successful incorporation of La ions into the MP structure. The bioactivity of the materials was assessed in vitro using simulated bodily fluid (SBF) at 37°C for a duration of 14 days in a shaker incubator (50 rpm). The SEM showed that a bone-like apatite layer formed quickly on the nanopowders' surface, proving that they have unique bioactive properties. The EDX spectra confirmed the presence of Ca, P, Mg, and La elements after immersion in SBF. The MP nanopowders, both with and without La doping, demonstrated the capacity to stimulate bone formation in a rat femoral bone defect model over a 28-day duration. Radiographic and histological studies showed that the La-doped MP nanopowders greatly improved bone repair and regeneration in comparison to the La-free nanopowders. Finally, the readily producible mesoporous MP nanomaterials, especially those with increased La doping (up to 7 wt%), exhibit significant potential for the restoration of large bone defects. Hence, fabricated nanopowders have immense promise for repairing bone criterion defects.