This dissertation investigated the potential impacts of changing climate, land cover, and sea level on the future stormwater budget and extreme runoff in coastal urban-natural environments. Two large-scale basins, namely Florida Southeast Coasts Basin (7117 km2) and Saint Johns River Basin (24928 km2), were selected for this study. The basins represented gradients in climate, land cover, and hydrology across the Atlantic coasts of Florida and the southeast U.S. Two mechanistic hydrologic models were developed for the basins using U.S. Environmental Protection Agency (U.S. EPA)’s Storm Water Management Model (SWMM) 5.1. The models were calibrated and validated with observed historical streamflow of the 2010s (2004-2013), computing the corresponding runoff volume as a historical reference. Runoffs for 2050s (2044- 2053) and 2080s (2076-2085) were quantified by incorporating climatic projections from 20 General Circulation Models (GCMs) and land cover projections from EPA under the Representative Concentration Pathways (RCP) 4.5 and 8.5 scenarios. Results suggested a predominant climatic control on potential runoff changes and high vulnerability in the coastal urban-natural environments. The relative increases in runoff were higher during the dry season and transitional months (October-May) than the wet season (June-September). Based on the basin-scale annual averages, runoff sensitivity to changes in rainfall was substantially stronger than that to changes in watershed imperviousness and evapotranspiration. Further, the concurrent changes in climate and land cover led to synergistic (stronger) nonlinear responses of runoff, compared to the linear summation of their individual effects. The dissertation also evaluated potential future changes in extreme events (rainfall and sea level) and associated runoff extremes across the two basins. Significance and direction of trends in 50-year annual maximum rainfall and sea level of 1-7 day durations for the 2000s (1964-2013) were evaluated by non-parametric Mann-Kendall test and Theil-Sen slope estimator. At the 5% level of significance, statistically insignificant mixed (increasing and decreasing) trends were noted in historical rainfall and significant increasing trends were observed in sea level. Accordingly, historical and future stationary design rainfalls and nonstationary design sea levels for 1-7 day durations with varying return periods (2, 5, 10, 25, 50, and 100 year) were estimated using Gumbel probability distribution, representing the family of generalized extreme value distributions. Based on the analyses, the extreme rainfalls and sea levels over various durations would substantially increase in the 2050s (2025-2074) and 2080s (2050-2099). Higher (than other locations) runoff increases were noted at and around the urban centers across the two basins. The projected extreme stormwater runoff under the changing regimes of extreme rainfall, sea level, and land cover suggested intensified risks of pluvial flooding and water pollution across the Atlantic coasts of Florida. The findings would guide stormwater management and ecosystem protection for coastal built and natural environments in the southeast U.S. and around the globe.