<p>Williamson nanofluids have been extensively studied due to their ability to model and enhance non-Newtonian fluid behavior in advanced heat- and mass-transfer applications. Motivated by this, the present work investigates the flow of a Williamson nanofluid over a stretching sheet in a porous medium, subject to Newtonian heating, chemical reactions, and a non-uniform heat source/sink. The governing momentum, energy, and species-concentration equations are transformed into a system of nonlinear ordinary differential equations via suitable similarity transformations. The resulting system is solved via the fourth-order Runge–Kutta method combined with the shooting technique. Graphical results illustrate the influence of key dimensionless parameters on the velocity and temperature fields. It is observed that increasing the Williamson fluid parameter from 0.1 to 0.5 and the permeability parameter from 0.2 to 0.6 reduces the maximum velocity by approximately 12% and 9%, respectively, while increasing the thermal boundary layer thickness by 8% and 6%. An increase in the chemical reaction rate from 0.1 to 0.4 decreases the surface temperature by about 7%. Furthermore, stronger non-uniform heat source/sink effects (from 0.2 to 0.8) combined with Newtonian heating raise the fluid temperature by nearly 10%, whereas increasing the Prandtl number from 0.7 to 7.0 enhances cooling by 15%. Quantitatively, the skin-friction coefficient decreases by 11% with the Williamson parameter and 8% with the permeability factor, while the Nusselt number increases by 12% due to stronger heat source/sink effects and Newtonian heating. These results provide detailed quantitative insight into the coupled effects of non-Newtonian behavior, chemical reaction, and thermal conditions in Williamson nanofluid flows.</p>