<p>This work investigates the heat transfer and three-dimensional magnetohydrodynamic (MHD) flow of a Casson-Williamson nanofluid over a stretching surface. To account for non-Newtonian behavior, the base fluid is modeled using the Casson-Williamson rheological framework, while the nanoparticle transport is described through the Buongiorno nanofluid model, incorporating the effects of Brownian motion and thermophoresis. The study considers the influences of internal heat generation, an applied magnetic field, activation energy and chemical reaction. Similarity transformations are employed to reduce the governing nonlinear partial differential equations to a system of ordinary differential equations, which are solved numerically using Runge–Kutta method. Key physical characteristics are thoroughly analyzed, focusing on temperature profiles and axial and transverse velocity fields. Quantitative analysis indicates that increasing the heat source parameter substantially enhances the thermal field, while higher activation energy promotes nanoparticle concentration. In contrast, stronger chemical reaction effects significantly suppress the concentration distribution. These findings provide valuable insights for engineering and industrial processes involving hybrid non-Newtonian nanofluid.</p>