Herein, activated carbon/epoxy composite was synthesized, in which waste mango leaves biomass were used as carbon source. To create a sufficient amount of porosity in the biomass, a simple process of carbonization followed by its activation was used. The morphology and porosity of activated carbon material were studied by SEM, TEM images and BET analysis, which confirmed the presence of macro and mesopores in the material. The activated carbon/epoxy composite having 3 mm thickness gave the highest Reflection Loss (RL) of -39.57 dB at 6.48 GHz. At a thickness of just 2 mm the maximum effective absorption bandwidth (RL < -10 dB) of 2.14 GHz was achieved. This excellent microwave absorption performance was attributed to large surface area due to porous and bowl type structures, which resulted in high impedance matching and subsequent attenuation of microwaves by interfacial and dipolar polarization. Present study provides porous activated carbon/epoxy composite that is cost effective, lightweight and has high structural stability for microwave absorption applications.