The Ten Thousand Islands region of southwestern Florida, USA is a major feeding and resting destination for breeding, migrating, and wintering birds. Many species of waterbirds rely specifically on marshes as foraging habitat, making mangrove encroachment a concern for wildlife managers. With the alteration of freshwater flow and sea-level rise trends for the region, mangroves have migrated upstream into traditionally salt and brackish marshes, mirroring similar descriptions around the world. Aside from localized freezes in some years, very little seems to be preventing mangrove encroachment. We mapped changes in mangrove stand boundaries from the Gulf of Mexico inland to the northern boundary of Ten Thousand Islands National Wildlife Refuge (TTINWR) from 1927 to 2005, and determined the area of mangroves to be approximately 7,281 hectares in 2005, representing an 1,878 hectare increase since 1927. Overall change represents an approximately 35% increase in mangrove coverage on TTINWR over 78 years. Sea-level rise is likely the primary driver of this change; however, the construction of new waterways facilitates the dispersal of mangrove propagules into new areas by extending tidal influence, exacerbating encroachment. Reduced volume of freshwater delivery to TTINWR via overland flow and localized rainfall may influence the balance between marsh and mangrove as well, potentially offering some options to managers interested in conserving marsh over mangrove.