Biochar can remove oppositely charged ionic species of Cd(II) and As(III)/As(V) from water but the co-adsorption mechanisms of the metal(loid)s are poorly understood. This study aims to unravel the co-absorption mechanisms of Cd(II) and As(III) by a Pennisetum sp. straw biochar (BC) modified with Fe-Mn oxide (FMBC). The maximum adsorption capacity of Cd(II) and As(III) by FMBC was 141.1 and 31.8 mg/g, respectively, which was 2.71 and3.89 times higher than that of BC. The adsorption of Cd(II) by FMBC demonstrated a homogeneous monolayer chemical process, while the As(III) adsorption presented a heterogeneous multilayer process. In the dual-adsorbate system, Cd(II) increased the adsorption of As(III) on FMBC by 5.1–123.9%, while As(III) reduced the adsorption of Cd(II) by 0.7–24.8%, indicating respective synergism and competition effect. Systematic co-adsorption experiments and adsorbent characterization results suggested that the synergistic mechanism occurred via electrostatic attraction and ternary surface complexation, whereas the competitive effect emerged due to complexing reaction with oxygen-containing functional groups of FMBC. The FMBC could be a potential adsorbent to adsorb Cd(II) and As(III) simultaneously.