Zn1 − xMgxO/Mn3O4/MnO (x = 0, 0.01, 0.03) nanocomposite samples were produced using the solid-state reaction at low temperature. The phase analysis for the synchrotron x-ray diffraction data for all samples was performed using the search match software. We determined the phase percentages using the Rietveld method. The crystallite sizes for all phases were determined. FTIR and Raman spectra confirmed the presence of ZnO, MnO and Mn3O4 phases in the formed samples. The Rietveld analysis-determined crystallite size anisotropy is evident in the rod-like form using the scanning electron microscopy technique. The absorbance and reflectance spectra of all samples were determined using UV–Vis diffuse reflectance technique. The optical band gap value depended on the amount of Mg in the samples. All samples have three optical band gaps assigned to ZnO, Mn3O4 and MnO phases. In comparison to higher energy photons, Zn1-xMgxO/Mn3O4/MnO nanocomposite samples showed a greater absorption rate for minimal energy photons. The results obtained suggest that Mg-containing materials are less likely to interact with gamma rays than to pass through them. The greatest fast neutron removal cross section (FNRCS) value was shown by the nanocomposite containing 1% of Mg. Our Mg-containing nanocomposite samples exhibit remarkable neutron shielding properties.