As key blue carbon ecosystems, mangroves store 70–90% of their carbon belowground as soil organic carbon (SOC). However, global spatial heterogeneity means that high SOC stocks do not consistently align with plant productivity. This mismatch calls for clarifying the drivers of SOC distribution and the conversion of inputs to long‑term soil storage. Here, we compiled a field‑based dataset of 795 sampling sites across 78 countries to assess the effects of climate, coastal environmental setting (CES), dominant genus, biomass input, and preservation conditions (represented by bulk density and theoretical tidal range) on SOC. Our results show that CES was the strongest individual predictor, explaining 28.9% of SOC variation, followed closely by preservation conditions (28.0%), whereas climate and biomass input explained only 7.3% and 10.2%, respectively. Adding CES to a climate‑only model raised R² from 0.071 to 0.297, and the full model including preservation reached 0.427. Structural equation modelling revealed that the overall effect of climate on SOC was counterbalanced by opposing influences on biomass production and direct soil processes. In contrast, CES exerted the largest total effect, primarily through enhancing carbon preservation. These findings establish geomorphic preservation, not plant productivity, as the primary control on long‑term mangrove SOC, with direct implications for blue carbon modelling and conservation. These global field data confirm that physical preservation conditions outweigh biomass input in governing SOC storage. Critically, they demonstrate that local in situ physical data are indispensable for refining Earth system models, as large‑scale models with coarse resolution fail to capture the fine‑scale topographic and hydrodynamic gradients that define intertidal carbon preservation.