Hybrid composites containing Cu nanostructures (NSs), reduced graphene oxide (RGO), and polystyrene (PS) were produced for electromagnetic shielding application. First, Cu NSs including nanoparticles (NPs), nanorods (NRs), and nanowires (NWs) were prepared by a green method using eucalyptus bark extract from the reaction of copper(II) chloride in ethylene glycol (EG) at 186 °C. The FE-SEM images, XRD patterns, and Raman spectroscopy were used to characterize these NSs. The RGO/PS composite was prepared by introducing RGO to the PS matrix. The tensile test revealed that by adding 10% of RGO to the PS matrix, the tensile strength of the RGO/PS composite increased by 13.8%. The thermal properties of the RGO/PS composite indicated that with the uniform incorporation of RGO on the PS matrix, the thermal stability of the polymer composite was elevated to 350 °C and the Tg to 112 °C. Conductive polymer nanocomposites (PNCs) were prepared by adding Cu NSs to the RGO/PS composite. The results showed that these PNCs had electromagnetic interference (EMI) shielding; the shielding effectiveness of PNCs containing 10% RGO and 25% Cu NWs was 36.0 dB at 8.0 GHz and 29.5 dB at 12.0 GHz. The predominant mechanism of protection in the frequency range and percentages of the filler under study was of the reflection type. Display omitted The Cu/RGO/PS polymer NCs have the ability of shielding against the interference of electromagnetic waves; so that the shielding effectiveness in the sample of polymer NCs containing 10 wt% of RGO and 25 wt% of Cu NWs were 36.0 dB at 8.0 GHz and 29.5 dB at 12.0 GHz. •RGO & Cu NSs were prepared by a green method using eucalyptus bark extract.•Adding 10% RGO, the tensile strength of RGO/PS polymer composite increased by 13.8%.•Thermal stability and Tg of Cu/RGO/PS NCs increased to 350 and 112 °C, respectively.•Conductive polymer NCs was prepared by adding Cu NSs to RGO/PS polymer composite.•Shielding effectiveness of Cu/RGO/PS NCs were 36 dB at 8 GHz and 29.5 dB at 12.0 GHz.