This study reports the synthesis and characterization of sustainable activated carbon–based composites derived from Arenga pinnata palm fiber and natural iron sand for X-band microwave absorption. The composites were prepared via a coprecipitation-assisted route using palm fiber–derived carbon and natural iron sand at mass ratios of 1:1, 3:5, and 5:3. Structural, morphological, and electromagnetic properties were evaluated using X-ray diffraction (XRD), scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM-EDX), and vector network analysis (VNA) in the 8–12 GHz range. XRD confirmed the formation of a dominant cubic spinel Fe₃O₄ phase with an amorphous carbon matrix, while SEM-EDX revealed porous morphologies with homogeneously distributed C, Fe, and O. The average particle sizes ranged from 0.207 to 0.241 μm. Microwave absorption measurements showed effective attenuation for all compositions, with the 3:5 composite achieving the best performance, exhibiting a minimum reflection loss of −12.31 dB at 10.54 GHz and an absorption efficiency of 87%. The enhanced absorption is attributed to the synergistic interaction between magnetic and dielectric loss mechanisms.