Background: Graphene-based nanomaterials possess unique optical, physicochemical and biomedical properties which make them potential tools for imaging and therapy. Manganese oxide nanoparticles are attractive candidates for contrast agents in magnetic resonance imagint (MRI). We used manganese oxide (Mn{}_3O{}_4) and highly reduced graphene oxide (HRG) to synthesize hybrid nanoparticles (HRG-Mn{}_3O{}_4) and tested their efficacy for photodynamic therapy (PDT) in breast cancer cells. Methods: The newly synthesized nanoparticles were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, UV-visible spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, thermogravimetry, and X-ray diffraction (XRD) analyses. We used standard protocols of cytotoxicity and PDT after exposing A549 cells to various concentrations of hybrid nanoparticles (HRG-Mn{}_3O{}_4). We also performed fluorescence microscopy for live/dead cellular analysis. A549 cells were incubated with nanoparticles for 24 h and stained with fluorescein diacetate (green emission for live cells) and propidium iodide (red emission for dead cells) to visualize live and dead cells, respectively. Results: The cell viability analysis showed that more than 98% of A549 cells survived even after the exposure of a high concentration (100 \mug/mL) of nanomaterials. These results confirmed that the HRG-Mn{}_3O{}_4 nanoparticles are nontoxic and biocompatible at physiological conditions. When the cell viability analysis was performed after laser irradiation, we observed significant and concentration-dependent cytotoxicity of HRG-Mn{}_3O{}_4 as compared to Mn{}_3O{}_4 nanoparticles. Fluorescence microscopy showed that almost 100% cells were viable when treated with phosphate buffered saline or Mn{}_3O{}_4 while only few dead cells were detected after exposure of HRG-Mn{}_3O{}_4 nanoparticles. However, laser irradiation resulted in massive cellular damage by HRG-Mn{}_3O{}_4 nanoparticles which was directly related to the generation of reactive oxygen species (ROS). Conclusions: HRG-Mn{}_3O{}_4 hybrid nanoparticles are stable, biocompatible, nontoxic, and possess therapeutic potential for photodynamic therapy of cancer. Further studies are warranted to explore the MRI imaging ability of these nanomaterials using animal models of cancer.