Tropical mangroves are highly carbon-rich coastal ecosystems that sequester atmospheric carbon dioxide in both aboveground biomass and belowground soil, playing a considerable role in climate change mitigation. However, the carbon storage potential of restored mangrove ecosystems in Sri Lanka remains largely understudied. This study examines how mangrove restoration and afforestation interventions contribute to the blue carbon pool. Restoration initiatives in the Chilaw lagoon began in 1994 using Rhizophora mucronata and R. apiculata. The study, conducted in a 25-year-old mangrove plantation covering approximately 0.25 ha, involved species identification, measurement of girth at brest height and height, and estimation of above and below-ground biomass and organic carbon content using species-specific allometric equations and wood density values based on established methodologies. Our findings indicate that favorable site conditions developed over 25 years have facilitated natural colonization of monospecific plantations by neighboring mangrove species, with Avicennia officinalis and Excoecaria agallocha as dominant colonizers, and others like Aegiceras corniculatum and Lumnitzera racemosa establishing to a lesser extent.The total organic carbon (TOC) content of tree biomass in 25-year-old planted mangrove stands was 271.97 Mg/ha, with a Simpson’s diversity index of 0.42. Notably, Rhizophora apiculata exhibited a higher TOC sequestration rate than Rhizophora mucronata, despite the latter being the most commonly used species in restoration projects in Sri Lanka. Among the species, A. officinalis exhibited a higher average TOC sequestration potential and sequestration rate, averaging 8.60 ± 1.6 kg/tree/year, followed by L.racemosa, R. apiculata, and R. mucronata. Our findings highlight that the species composition and stand structure of later-colonizing species significantly enhance the carbon sequestration function of restored mangrove ecosystems. These results highlight that although a single mangrove species was initially planted, suitable environmental conditions such as particularly substrate quality supported by high seed rain enables natural colonization of the area by other mangrove species. This suggests that monospecific plantations established in appropriate habitats, can facilitate the development of multispecies stands with time, tthus improving carbon sequestration capacity and ecosystem resilience.