Strongly absorbing, thin, wide-bandwidth microwave absorption materials are highly desirable. Meanwhile, sustainable, abundant and low-cost wood with a highly porous 3D structure provides an excellent network and many other benefits, which inspired us to design new absorption materials. In this paper, we employed wood and KMnO₄ as precursors to successfully prepare biomass-derived carbon/MnO nanorod (BDC/MnO NR) composites through a low-cost method combining hydrothermal treatment, carbonization and annealing. The minimum reflection loss (RL_min) of the BDC/MnO NR composites was −58.5 dB at a thickness of only 1.65 mm (for BDC, −15.2 dB at 5 mm), and the covered effective absorption bandwidth reached 14.5 GHz (for BDC, 3.4 GHz). The improvement in absorption performance can be attributed to the promotion of the impedance match and attenuation constant. Compared with previously reported similar microwave absorption materials, our products are thinner and have a stronger capacity, indicating that the BDC/MnO NR composites could be promising candidates as high-performance absorbers.