This study aimed to investigate the efficiency and mechanisms of a two-stage membrane biofilm reactor (MBfR) for the simultaneous advanced treatment of landfill leachate and the desulfurization and decarbonization of landfill gas (LFG). The system consisted of a pre-stage MBfR for partial nitrification and a post-stage MBfR for nitrogen removal and gas purification. During 280 days of operation, the integrated system demonstrated outstanding performance in removing organic matter and nitrogen from leachate, along with efficient purification of LFG by eliminating hydrogen sulfide (H2S) and carbon dioxide (CO2), thereby upgrading its methane (CH4) content. By maintaining low dissolved oxygen (DO) levels (0.05–0.1 mg/L) in the pre-MBfR, a nitritation efficiency exceeding 85% was achieved, enabling effective partial nitrification. This provided an optimal NO2-/NH4+ ratio for the subsequent anaerobic ammonium oxidation (anammox) process coupled with denitrifying anaerobic methane oxidation (DAMO) in the post-stage MBfR. The system attained average removal rates of 95% for chemical oxygen demand (COD), 99% for NH4+, and 99% for total nitrogen (TN), indicating highly efficient carbon and nitrogen removal from landfill leachate. The high removal efficiency was maintained even under a high nitrogen loading rate of 8,000 mg N/L·d and a low hydraulic retention time (HRT) of 0.25 days, demonstrating the process robustness and operational stability. Concurrently, the post-stage MBfR achieved significant LFG purification by reducing CO2 and H2S concentrations to below 0.2% and approximately 5%, respectively, while increasing the CH4 content to approximately 80%. This upgrade in LFG quality enhances its potential for energy recovery and reduces environmental impacts. Microbial community analysis revealed the enrichment of electroactive microbes, such as Methanothrix and Thiobacillus, which played crucial roles in methane production and sulfur-based autotrophic denitrification. The presence of these microorganisms indicates the occurrence of synergistic biochemical reactions including sulfur cycling, methane metabolism, and nitrogen removal. Further multi-omics analysis indicated that the synergy between denitrifying anaerobic methanotrophic archaea and methanogenic archaea likely facilitated direct interspecies electron transfer (DIET), promoting the reduction of CO2 to CH4. Metagenomic and metatranscriptomic analyses confirmed the high activity of key genes and pathways involved in electron transfer and methane metabolism. Additionally, the high organic loading in the anaerobic environment consumed substantial protons, increasing system alkalinity and enhancing the absorption of CO2 into the liquid phase. This mechanism further contributed to the reduction of CO2 and enrichment of CH4 in the upgraded LFG. The results also showed that the system offers significant economic and environmental benefits by reducing the need for external carbon sources and alkaline chemicals, both of which are typically required in conventional treatment processes. This integrated approach presents a sustainable alternative for simultaneous waste stream management and energy recovery from landfill operations.