Coastlines and their mangrove forests change over time under the influence of human and natural drivers. To design appropriate mangrove reforestation interventions, we use the Vietnamese province of Soc Trang, at the Bassac River mouth, as a case study to understand coastal zone changes. Our research, covering 1904–2007, is based on historical material from the French colonial period (topographic maps, reports), satellite images, and onsite interviews with key informants. Since 1904, the coastline and mangrove forests have changed significantly, including a sequence of deforestation and reforestation in some areas, changes in tree species composition, transformation of the coastline landscape from sand dunes to mangrove forests, and large-scale accretion at the river mouth. The natural processes of accretion and erosion have changed over time for the same area in Vin Chau District, thus influencing mangrove cover and reforestation programs. Damage to the mangrove forest during the Vietnam War due to defoliants was localised to specific areas along the coastline, and damaged trees were later cut for local use. Deforestation for fuelwood, expansion of farming areas, access rights, and usage of the mud flats during the French colonial period, followed by reforestation that modified the original species composition, are the main drivers of coastline changes. These drivers influence the coastline and mangrove cover dynamics in different ways. The knowledge of historical processes and coastal dynamics is important in developing climate change adaptation strategies, which combine such site-specific measures as effective mangrove protection and management, mangrove rehabilitation, and engineering measures.