Afforestation is a process used in recovering the global mangrove cover. Succession or development in mangrove communities during mangrove afforestation has rarely been described yet is important for understanding the potential of afforestation in creating diverse and functional mangroves. Since 1966, the Government of Bangladesh has created nearly 280 km2 of plantation mangroves (7.5% of the extent of the Sundarbans mangrove forest and 0.21% of the global mangroves extent) on the Delta of the Ganges, Brahmaputra and Meghna Rivers, located east of the Sundarbans mangroves. Here, we investigated the development of these plantation mangroves, assessing their community structure and species richness over a 40-year chronosequence in comparison to natural mangroves in the same region. Tree communities in the plantations reached a maximum mean aboveground biomass (AGB) of ∼157 ± 13 Mg dry weight ha−1, achieving equivalence in tree aboveground biomass and tree density (255 ± 2 individuals ha−1) to natural mangroves by 20 and 39 years respectively. Our dataset from plantations aged between 10 and 42 years indicated that structural complexity (Ic) was higher in older plantations than in younger plantations. We found that the tree community structure of older mangrove plantations was more diverse in the eastern region of the delta compared to the central and western regions, and the landward plantations were more diverse than the plantations fringing the oceans. One of the originally planted mangrove species, Sonneratia apetala, was dominant in these plantations. However, we recorded 8 mangrove tree species from 5 genera in the plantations, despite only two species (S. apetala and Avicennia officinalis) being planted. Our data indicated that while the tree structure of the established mangrove plantations was similar to natural mangroves within 42 years, equivalent species richness may not be reached for more than 40 years. Therefore, adoption of management to enhance natural successional or development processes in afforestation programs, for example by planting a diverse mix of species, could improve the effectiveness of afforestation in conserving biodiversity.