Lithium (Li) is used in various applications involving pharmaceuticals, textile dyes, and batteries. Therefore, the demand for environmentally friendly and effective materials for Li uptake and recovery continues to increase. Herein, rice husk (RH) and coconut shell (CS) biomasses were used to fabricate honeycomb-networked biochar (BC) precursors via slow pyrolysis. RHBC- and CSBC-based MnO composites were synthesized by depositing MnO in various ratios onto RHBC and CSBC by varying the KMnO concentration (2%, 3%, and 4%), followed by simple ultrasonication and heat-treatment methodologies. The structural and physicochemical properties of all of the fabricated composites were analyzed using several different instrumental methods. The batch adsorption experiments were performed for comparative Li-adsorption studies of RHBC-Mnx and CSBC-Mnx composites by optimizing several parameters (pH, adsorbent dose, Li initial concentration, and contact time). The comparative adsorption analysis revealed that the RHBC-Mnx composites exhibited stronger Li-adsorption ability than the CSBC-Mnx composites and that increasing the MnO deposition to 3% in both cases led to maximum Li adsorption capacities (62.85 mg g and 57.8 mg g), respectively. The kinetic studies show that Li adsorption proceeds through the pseudo-second-order mechanism. Li recovery was successfully carried out using HCl (eluting agent), thereby demonstrating the benefits of synthesized composites at the industrial scale. The current work indicates that the fabricated RHBC-Mnx and CSBC-Mnx composites may have potential for use as economical composites in eco-friendly applications such as Li adsorption and recovery from aqueous media.