The management of landfill leachate continues to provide a significant environmental issue owing to its elevated organic load, low biodegradability, and the presence of hazardous substances. This study investigated the enhancement of biogas production efficiency through anaerobic co-digestion of organic waste with different proportions of mature landfill leachate (ML) and pretreated mature leachate by the adsorption process (TA). Durian shell-derived activated carbon was prepared and applied to pretreat mature leachate prior to anaerobic digestion. In experiment I, the liquid mixture of distilled water (W) and mature leachate (ML) was varied as 100:0, 75:25, 50:50, 25:75, and 0:100, which was based on volume. Biochemical methane potential (BMP) assays were conducted under mesophilic conditions (35 ± 1°C) for 50 days to evaluate methane yield and organic matter removal efficiency. The result of this study indicates that the liquid mixture of 75% distilled water and 25% mature leachate (W75:ML25) had the highest methane production and organic removal efficiency. Adding mature leachate to the system could reduce the lag phase and improve buffering capacity. However, higher mature leachate proportions (>25%) reduced methane yield due to inhibitory substances. In experiment II, the liquid mixture ratios of mature leachate (ML) to treated mature leachate by adsorption (TA) were varied as 100:0, 75:25, 50:50, 25:75, and 0:100, based on volume. The liquid mixture of 25% mature leachate and 75% treated leachate by adsorption (ML25:TA75) produced the highest methane accumulation and organic removal efficiency. Pre-treatment of mature leachate by the adsorption process could improve leachate quality and further enhanced anaerobic digestion performance.