A study is performed on two-dimensional flow and diffusion of chemically reactive species of Casson fluid from an unsteady stretching surface in porous medium in the presence of a magnetic field. The boundary layer velocity, temperature, and concentration profiles are numerically computed for different governing parameters. The paper intends to show unique results of a combination of heat transfer and chemical reaction in Casson fluid flow. The resulting partial differential equations are converted to a system of ordinary differential equations using the appropriate similarity transformation, which are solved by using the Runge-Kutta-Fehlberg numerical scheme. The results in this work are validated by the comparison with other authors.