The electromagnetic interference (EMI) shielding effectiveness of the epoxy polymer composite reinforced with carbon fiber mat (CFM), short carbon fiber (SCF), and multi-walled carbon nanotube (MWCNT) is studied in the X-band (8–12 GHz). Single-layer unidirectional CFM of 30 wt% epoxy composite with 0° and 90° orientations resulted in EMI shielding of 5 dB and 29.6 dB, respectively. To avoid the influence of CFM orientation on the EMI shielding, SCF of distinct weight percentages (1 and 5 wt%) with fixed weight percentage (1 wt%) of MWCNT are reinforced into epoxy matrix. The EMI shielding provided by 2 mm thick epoxy composite reinforced with 5 wt% SCF along with 1 wt% MWCNT (CNT-SCF5) is 35.4 dB. The microstructural study revealed uniform dispersion and better adhesion of MWCNT and SCF in the epoxy matrix. The dielectric properties of the fabricated samples indicated an increase in permittivity, electrical tanδ, and conductivity with the grading of SCF and MWCNT. The mechanical properties such as tensile strength and strain of the CNT-SCF5 showed an increment in ultimate tensile strength and Young's modulus. The homogeneous CNT-SCF5 epoxy composite with enhanced EMI shielding ability and mechanical properties could be utilized as an efficient EMI shielding material. © 2022 Society of Plastics Engineers.