In this paper, we have studied the effect of heat source/sink on unsteady Casson nanofluid past a stretching surface with mixed convection inclined magnetic field and nonlinear thermal radiation numerically. Brownian and thermophoresis effects are studied in this nanofluid model (Buongiorno’s). The governing momentum, energy, and concentration equations are PDEs that are changed into ordinary differential equations by means of suitable transformations. The fourth-order R-K method with shooting technique is adapted to yield the results of this work. The velocity, thermal, and concentration profiles are discussed with the several physical parameters. Also, skin friction, the Nusselt number, and the Sherwood number are examined with the help of the table. It is found that the enhancing value of the unsteady parameter and heat sink parameters reduce the fluid temperature, and the enhancing value of the Casson parameter and heat source parameters increase the fluid temperature. The increasing value of the inclined magnetic field parameter enhances the thermal boundary layer thickness.