Magnetic iron oxide nanoparticles (MNPs) were synthesized using Albizia adianthifolia leaf extract as reducing and protecting agent. Colour changing, UV-Vis spectrum, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) confirmed the biosynthesis and characterization of MNPs. The XRD pattern revealed that MNPs are crystalline in nature. FT-IR spectral analysis showed that MNPs was capped with plant constituents. From SEM analysis, the MNPs were generally found to be spherical in shape and the size was ranged 32-100 nm. Free radical scavenging potentials of the MNPs against DPPH were confirmed based on its stable anti-oxidant effects. The synthesized MNPs were used to capture Staphylococcus aureus under the magnetic field effect. Further, it was observed that the MNPs are able to exert cytotoxic effect towards human breast (AMJ-13) and (MCF-7) cancer cells. The anti-proliferative effect of this treatment is due to cell death and inducing apoptosis. Mitochondrial membrane potential, acridine orange-propidium iodide staining assays as well as single cell and DNA gel electrophoresis analyses indicated that MNPs induce cell death only by apoptosis. The findings of present study suggest that the MNPs might be used for medicinal applications particularly for cancer therapeutics.