Watershed models are typically calibrated and validated with the same land use and land cover (LULC) dataset and later used in assessing impacts of changing LULC, such as urbanization, on hydrology and/or water quality. However, their performance in predicting water quality/quantity in response to changing LULC is rarely assessed. The main objective of this paper was to explore the performance of the soil and water assessment tool (SWAT) in predicting water quality and quantity in response to changing LULC in a coastal watershed in Alabama, USA. Using the 1992 LULC as the input, the model was calibrated and validated for flow for the period 1990–1998, and for total suspended solids (TSS), nitrate (NO3−), and organic phosphorus for the period 1994–1998 at several sites within the watershed. The model was then driven with the 2008 LULC data and its performance in predicting flow and TSS, NO3−, and total-P loads during the period 2008–2010 was evaluated (post-validation). SWAT showed good performance in predicting changes in flow and water quality during the post-validation period. The study also highlighted the importance of using the most up-to-date LULC data for effectively predicting the impacts of LULC changes on water quality.