The Sundarbans, the world’s largest contiguous mangrove ecosystem spanning India and Bangladesh, are critical hubs for biodiversity conservation and climate resilience. This review synthesizes quantitative assessments of biodiversity across floral, faunal, and microbial communities, revealing 30 mangrove species, 4037 animal species, and diverse microbial assemblages dominated by the phyla, Proteobacteria and Firmicutes. The ecosystem demonstrates remarkable climate resilience mechanisms, including carbon sequestration (22.8–24.7 Mg C ha−1 in aboveground biomass), coastal protection via 50–90% wave energy attenuation, and adaptation to sea-level rise through sediment accretion rates of 3.8–7.6 mm yr−1. However, the Sundarbans face unprecedented conservation challenges from climate pressures, such as a 3.14 mm yr−1 sea-level rise and 15–20% salinity increases since 2000, alongside anthropogenic stressors causing a 40% reduction in mangrove cover in heavily impacted zones. These stresses on biodiversity are exacerbated by governance flaws, including institutional disunity and inadequate transboundary coordination. Management solutions include technological integration using UAV monitoring and remote sensing, ecological restoration using community-based methods that achieve 70–80% sapling survival rates, and sustainable livelihood initiatives that increase income by 15–20% while reducing resource exploitation. Strong cross-border procedures should be established, co-management frameworks consolidated, and mangrove conservation incorporated into Nationally Determined Contributions. To preserve the ecosystem’s crucial role in regional climatic stability and biodiversity protection, the findings highlight the urgent need for transboundary cooperation, adaptive governance, and science-policy integration in the conservation of the Sundarbans.