Rapid population growth and economic change on the tropical islands of Mauritius have led to one of the highest rates of urban build-out in the world. Pressure on many of the island's natural features and resources increasingly risks further degradation to the environmental services that they provide to the country. Fourteen types of marine and terrestrial environmentally sensitive areas (ESAs) are critical to the nation's sustainable development. Twelve of these ESA types are currently at risk of degradation, owing to their spatial proximity to built-up areas (BUAs) and current use designation. There was a bimodal distribution in proximity; eight of the 12 ESA types analysed had an area-weighted modal peak < 500 m from the nearest BUA, and four ESAs had a modal peak 2–3 km from the nearest BUA. Six coastal and marine ESAs had limited protection from urban expansion and over-use. The Mauritian experience reflects trends that are emerging across many tropical developing countries, where the bulk of future global growth in urban area is expected to occur. The approach detailed in this case study is replicable and may be useful in assessing degradation risk as a result of urban expansion in other island countries.