Mumbai, a financial center of India located in the western coastal region of India, has mangrove forests. There are long-settled fisherfolk called ‘Koli’, and mangroves are a significant part of the lives of the Kolis. Mangroves and the Kolis are now under increasing pressure from rapid urbanization. The objective of this study is to declare the status of the mangroves, mangrove habitats, and the Kolis in Mumbai, and to understand the impact of urbanization on them and the relationship between mangroves and the Kolis. The current mangrove habitats and mangrove status in the study sites were declared by water quality surveys and stable isotope ratios of carbon and nitrogen. The mangrove habitats in the study areas were degraded, and nitrate concentrations were higher during the rainy season than during the dry season, indicating that the mangrove habitat was severely polluted by sewage. The nitrogen stable isotope ratios of leaves were increased as nitrate concentration and salinity in the water samples were increased. The nitrogen stable isotope ratios of leaves were indicated to reflect the status of mangrove habitats. The carbon stable isotope ratios of tree-rings samples showed seasonal variations. The stable isotope ratios of tree-rings samples were indicated to reflect the environmental change over time. The status of the Kolis in the study sites were clarified by interviewing. We interviewed 31 Kolis and all Kolis answered the quantity of the fish catch at one time had decreased and most of the Kolis felt their relationship with mangroves had changed these days. This study attempted to when and how the relationship between the Kolis and mangroves has changed through ecological surveys and sociological interviews.