Abstract: This study aims to develop a high-performance, low-cost low-frequency electromagnetic (EM) wave absorbing material to address the growing issue of low-frequency EM pollution caused by devices such as 5G/6G, radar, and WiFi. By combining hydrothermal synthesis and high-temperature carbonization, a ternary composite material, MnFe₂O₄/C/CG (MFC), was successfully prepared with coal gangue (CG) as the matrix. The CG and the surface-deposited carbon layer possess abundant oxygen-containing functional groups and defect structures, which not only provide heterogeneous interfaces and enhance dielectric loss but also improve the dispersion of MnFe₂O₄ particles. Meanwhile, the MnFe₂O₄ particles contribute crucial magnetic loss capability, achieving a magneto-dielectric synergy. The results show that the MFC-2 sample exhibits strong low-frequency wave-absorbing performance in the C-band, achieving the minimum reflection loss of −57.82 dB and an effective absorption bandwidth of 3.54 GHz at a thickness of 4.86 mm, which covers the N79 band and 88.5% of the C-band. Its excellent performance is attributed to favorable impedance matching and a magneto-dielectric synergistic loss mechanism involving interfacial polarization, dipole polarization, and magnetic loss. This work provides a feasible strategy for developing high-performance, low-cost low-frequency wave-absorbing materials using bulk solid waste CG.