The Asian Swamp Eel (Monopterus albus/javanensis) is a synbranchid fish introduced to Florida in the 1990s that subsequently invaded the sub-tropical wetlands of southern Florida around 2005. Eel establishment has been linked to declines in several species of aquatic animals. We quantified gut contents of swamp eels across multiple Everglades regions to assess the energetic importance and relative vulnerabilities of different prey types. We quantified the energetic importance of prey types across eel size (ontogeny) and populations from regions with different establishment histories (recently invaded to well-established) and calculated prey selectivity for 12 larger collections (N≥10) of eels by comparing proportional prey use to prey availability using paired field density data. We dissected 843 eels (42–924 mm total length; TL) collected over 15 years from 22 sites over approximately 3000 km2 of wetland and 79% (N=662) contained at least one identifiable prey animal. We expected eels to show an ontogenetic dietary shift, with smaller eels consuming more insects and small crustaceans and that species that have disappeared from the wetlands (e.g., crayfish) would be both important and selected in recently invaded wetlands. Eels consumed at least 44 prey species and showed ontogenetic diet shifts; smaller eels (<250–300 mm TL) consuming more amphipods and coleopterans, while larger eels consumed more crayfish. Odonates (dragonfly naiads), crayfish (Procambarus spp.), fish (cyprinodontiformes), grass shrimp (Palaemonetes paludosus), and hemipterans were energetically important prey. Recently established populations relied more on crayfish, while longer-established populations consumed more fish. Crayfish and odonates were consistently selected prey where they were present, while fish, shrimp, and hemipterans were generally under-represented in the guts of eels. Asian Swamp Eels are generalist predators that disproportionately consume slow-moving crawling macro-invertebrates.