The present work is focused on room-temperature reduction by subjecting reduction of an aqueous KMnO4 and KMnO4/graphene oxide (GO) dispersion by Fehling solution B in one step to form δ-MnO2, exhibiting hierarchical nanoflowers and their nanocomposites (δ-MnO2/reduced graphene oxide). This was followed by their characterization by X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), surface area measurements, and DC conductivity. The effect of the concentration of a reducing agent on the morphological evolution of δ-MnO2 has also been proposed. Further, these materials were also investigated for their performance in electromagnetic interference (EMI) shielding efficiency (SE) in the frequency range of 2–8 GHz. These findings showed the highest total shielding efficiency (∼39 dB) of the δ-MnO2/reduced graphene oxide (RGO)-1.0 nanocomposite consisting of 5.5 wt % RGO following reflection as a dominant mechanism. Such excellent performance was attributed to the poor impedance matching between MnO2/RGO, the formation of an interconnected conducting network, and interfacial polarization. It is anticipated that δ-MnO2/RGO nanocomposites synthesized by a simple room-temperature reaction in one step could be promising candidates as lightweight and high-performance EMI shielding materials for multifaceted applications.