A modified version of Hummers’ process was used to synthesize graphene oxide (GO), whereas solar radiation was used to create reduced graphene oxide (rGO) and MnO2/rGO (1%, 3%, 5%) nanostructures were synthesized using a facile hydrothermal method. The X-ray diffraction (XRD) pattern revealed the tetragonal structure of α-MnO2 and determined the crystallite size of MnO2/rGO as 14.36 nm. A scanning electron microscope (SEM) analysis confirms the growth of MnO2 nanorods on the rGO surface. MnO2/rGO (1%, 3%, 5%) contains elements such as carbon, oxygen and manganese, as verified by energy-dispersive X-ray analysis (EDAX). When the rGO percentage was increased, the surface area of MnO2/rGO was decreased to 16.340 m2/g, which enhances the interaction between A549 cells, according to Brunauer–Emmett–Teller (BET) analysis. The allotropes of carbon and the functional groups were examined using Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). A (dimethyl thiazolyl tetrazolium bromide) MTT assay was used to assess the cytotoxicity of GO, rGO and MnO2/rGO (1%, 3%, 5%) nanostructures against the human lung adenocarcinoma cell line (A549) and human lung epithelial cell line (L132). The half maximum inhibitory concentration (IC50) value for MnO2/5% rGO towards A549 cells was determined to be 120 μg/mL, which shows appreciable results when compared to GO (180 μg/mL), rGO (160 μg/mL), MnO2/1% rGO (180 μg/mL) and MnO2/3% rGO (160 μg/mL). The morphological changes in A549 cells were investigated using an AO/EtBr dual stain, the intracellular reactive oxygen species produced were evaluated using a DCFH-DA stain and the fragmented nuclei were examined using a Hoechst stain. According to the findings, the synthesized MnO2/rGO nanocomposites can be efficiently used for cytotoxicity against A549 cells and possess significant potential for the human lung cancer cell line.