Microwave shielding properties of chemically synthesized MnO2 decorated graphene nanoribbons (GNR) is reported for the first time. The nature of MnO2 decoration on the GNR has been investigated using scanning electron microscope, X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. The electromagnetic interference (EMI) shielding effectiveness of this material was investigated in the microwave region (Ku-band, 12.4-18 GHz). The presence of MnO2 on GNR enhances the interfacial polarization, multiple scattering, natural resonances and the effective anisotropy energy, which leads to absorption, dominated high shielding effectiveness of -57 dB (blocking > 99.9999% radiation) by 3 mm thick sample. Dielectric attributes (ε' and ε'') is evaluated to understand the mechanism of excellent shielding effectiveness. The material shall be excellent choice for radar absorbing applications.