Polymer blends are at the forefront of research especially in the field of Electromagnetic Interference (EMI) shielding because of their versatile properties such as ease of processability, economic viability and high strength to weight ratio. Herein, we have attempted to design lightweight blend composites consisting of multiwalled carbon nanotubes (MWNTs) and nickel ferrite (NiFe2O4) nanoparticles. A unique approach was adopted here to prepare ultra-thin (500 μm), flexible, lightweight composite membranes wherein hierarchical nanoporous structures were initially developed by crystallization induced phase separation in a classical upper critical solution temperature (UCST) pair Polyvinylidene fluoride/poly methyl methacrylate (PVDF/PMMA) and subsequently etching out the PMMA phase. In the next step, functional nanoparticles were trapped in the pores by facile vacuum filtration approach. This unique approach led to the fabrication of nanoporous composite membranes which otherwise is difficult to process using conventional techniques. The composite membranes show high magnetic permeability and high electrical conductivity; the two key requirements for effective shielding of electromagnetic (EM) radiation. A significant improvement in shielding effectiveness (SE) was achieved using these token composite membranes. For instance, porous PVDF composite membranes containing 3 wt % MWNTs (with a thickness of 500 mm) showed an SE of 8 dB which enhanced significantly to 27 dB for composite membranes wherein NiFe2O4 is trapped in the pores. More interestingly, the mechanism of shielding was driven by absorption (nearly 80%) through synergistic properties of interconnected MWNTs and NiFe2O4 nanoparticles. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.