Abstract: Mangroves are a pivotal component of blue-carbon ecosystems, and the stability of soil organic carbon (SOC) in these systems exerts a critical control on their carbon-sink function. Along a latitudinal gradient from Fujian to Guangdong and Hainan, we collected surface (0–20 cm) and subsurface (40–60 cm) soils dominated by two representative mangrove species, Kandelia obovata and Avicennia marina. After 90 d of laboratory incubation, cumulative SOC mineralization (used as a proxy for SOC stability) was measured and related to different soil carbon fractions. Results showed no significant difference in cumulative SOC mineralization among latitudes or plant communities. SOC in deeper layers was more stable than in surface layers. Fine-particle content was positively correlated with the proportion of mineral-associated organic carbon (MAOC) to SOC, whereas the MAOC/SOC ratio was negatively correlated with cumulative mineralization. On average, MAOC accounted for 54.00% of SOC, whereas microbial residue carbon (MRC) represented only 14.27%. SOC was significantly correlated with both particulate organic carbon (POC) and MAOC, but the relationship was stronger for POC. These findings suggest that SOC stability in mangroves is primarily governed by soil texture, and the influence of latitude and community type requires further investigation. Microbial residue carbon is unlikely to be the main source of stable MAOC, whereas the labile POC fraction may be a key driver of SOC accumulation in mangrove soils.