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Information on agricultural Best Management Practices (BMPs) and their effectiveness in controlling agricultural non-point source pollution is crucial in developing Clean Water Act programs such as the Total Maximum Daily Loads for impaired watersheds. A modeling study was conducted to evaluate various BMPs including pasture planting, nutrient management, brush management, clearing and range planting, prescribed grazing, critical area planting, conservation cropping, contour farming, terrace, ponds, grade stabilization structures, and grassed waterways implemented in a 280-km2 Mill Creek Watershed in north-central Texas. The main objective of this study was to assess the long-term impacts of BMPs, at both field and subwatershed levels, on surface runoff, sediment, and nutrient losses using the Agricultural Policy/Environmental eXtender (APEX) model. Considering all BMPs, average annual reductions in runoff, sediment, total nitrogen (TN), and total phosphorus (TP) at the field level ranged from 0 to 52%, 36% to 99%, 0 to 96%, and 15% to 92%, respectively, reflecting the variability in topography, soils, landuse, climate, and relative magnitude of these constituents entering the field from upstream contributing area. However, at the subwatershed level, the reductions only ranged from 2.9% to 6.5% in runoff, 6.3% to 14.8% for sediment, 11% to 15.1% for TN, and 6.3% to 8.6% for TP. The impacts of BMPs on water quality varied depending on the type of practice and its location in the landscape. This study also showed that reduction in sediment at the watershed outlet was proportional to the area treated with BMPs.