Utilization of flyash which is produced at large scale in coal based thermal power plant is a challenge. In this regard, our investigation provides distinctive way of utilizing flyash for designing and preparing high-performance EMI shielding materials. Herein, we report synthesis and characterization of multiwalled carbon nanotubes (MWCNT) based multiphase composites. The multiphase composites were synthesized by in-situ co-precipitation with conductive filler (MWCNT) and magnetic filler (Ferrofluid). Scanning electron microscopy results confirm the presence of fly ash particles covered with Ferrofluid nanoparticles along with MWCNTs. Multiphase composites show total shielding effectiveness of 48 dB (>99.998 % attenuation) in the Ku-band (12.4–18 GHz) frequency range. The electromagnetic attributes, dielectric and permeability parameters have been calculated from the measured scattering parameters (S11, S22, S12, S21) using the Nicolson–Ross–Weir algorithm. The synthesized multiphase composites were characterized using XRD, FTIR, VSM and SEM. The results suggested that the MPC composites showed great potential as a radar absorbing material.