Biomass-based porous magnetic nanocomposites were prepared for microwave absorption in the X-band range. Fe3O4 nanoparticles were precipitated on activated carbon (AC) derived from cypress cones, creating composites with carbon contents ranging from 15 to 85 % by weight. Absorption experiments revealed that 15 % wt. nanocomposite exhibited optimal performance, showing a minimum reflection loss (RLmin) of −16.3 dB at a thickness of only 2.5 mm. Notably, this composites efficient absorption bandwidth covered 80 % of the X-band at this thin dimension. This study highlights the potential of this nanocomposite as an effective electromagnetic absorption material. Cole-Cole diagrams confirmed the multiple polarization and conduction losses. The impedance mismatch plot and attenuation constant represent significant impedance matching with free space and high dissipation, which improves the performance of nanocomposites with a ferrite/AC ratio of 85:15.