Magnetic bimetallic nanoparticles find many industrial and clinical applications in the field of water treatment, antibacterial and anticancer activities. Therefore, the current article reports green synthesis using oleo-polyol as a surface modifier and synthesis agent for bimetallic magnetic magnesium ferrite nanoparticles. The role of hydroxyl functionality of castor oil (a natural polyol) on the enhancement of structural, morphological, magnetic, and particle size properties has also been discussed. These properties were characterized using FTIR, XRD spectroscopy, SEM, AFM, and TEM microscopy, Brunauer− Emmett−Teller (BET), and vibrating sample magnetometer (VSM) techniques. The effect of calcination temperatures (600−900 °C) on particle size (23−40 nm to 500−600 nm), crystallite sizes (73.15−292.67 nm), and saturation magnetization (20.87, 23.07, 32.39, and 33.13 emu g −1 ) was analyzed. The influence of calcined temperatures on the anticancer activity of these nanoparticles has also been investigated in vitro using lung cancer cells (A549). Their biocompatibility, cytotoxicity, flow cytometry, and statistical analysis against lung cancer cells (A549) have been discussed. The green synthesis of magnesium nanoferrite particles using natural polyol and their application as anticancer agents against lung cancer cells (A549) have not been reported previously. They have exhibited far superior IC 50 values and anticancer activity as compared to other reported metal oxides and magnesium oxide nanoparticles.