Introduction of species by humans breaks down biogeographic boundaries and results in the homogenization of species composition, yet empirical tests of this impact in marine forest ecosystems are still scarce. Large-scale planting aimed at reversing losses of mangroves has been the dominant strategy for mangrove restoration adopted by many organizations in the past decades, but there is a lack of quantitative understanding of the impacts of such large-scale plantings on mangrove biogeography. Here we used data collected before and after large-scale planting to compare the species richness and compositional similarities among 72 mangrove sites over a biogeographic scale (18–28°N) in China. After the large-scale planting, 15 of the mangrove species spread toward the higher latitudes, reflecting the geographical barriers of the mangrove plants have been broken. Local species richness of mangrove increased by 44.82% and biogeographic compositional similarity of mangroves increased by 13.33%, reflecting large-scale introduction and planting increase local diversity of mangrove but enhance biological homogenization. The dispersal limitation of mangrove communities reduced by 11.1%, which indicates that the community assemblage process of mangrove changed obviously. Worryingly, two alien species, Sonneratia apetala and Laguncularia racemosa, have dispersal across the biogeographic scale studied, reflecting an increase in the risk of biogeographic invasion. It is expected that biological homogeneity and species invasion will further influence the functional biogeography of mangroves. Our results highlight that mangrove biogeography is defined by human activities in the Anthropocene.