In this study, expanded graphite (EG) was decorated with manganese oxide nanoparticles (MONPs) by the hydrothermal method, and the newly formed composite (MONPs-EG) was applied as adsorbent for the removal of heavy metals from aqueous solutions. The comparative and competitive adsorption of Pb²⁺ and Ni²⁺ (0.01–1.00 mM) on MONPs-EG was investigated. Data from isothermal adsorption of single and binary systems suggested that both Pb²⁺ and Ni²⁺ were well described by the Langmuir isotherm, and the maximum adsorption capacities at 298 K were calculated at 0.278 and 0.113 mmol/g for Pb²⁺ and Ni²⁺, respectively. In binary systems, a dramatic decrease in adsorption capacity of Ni²⁺ was observed, while the adsorption capacity of Pb²⁺ was almost stable, indicating the favorable adsorption of Pb²⁺ over Ni²⁺ onto the prepared adsorbent. Kinetics studies of single and binary systems showed that a pseudo-second order model could explain the adsorption processes well. Thermodynamic analysis results demonstrated that the adsorption of these metal ions on the prepared adsorbent is spontaneous and exothermic in nature. The adsorption capacity of MONPs-EG increased significantly in the presence of humic acids. Overall, the results of this study suggest that MONPs-EG can be used effectively as an adsorbent for heavy metals removal from aqueous solutions.