The present study deals with the mixed convection MHD flow of a Casson nanofluid over a nonlinear permeable stretching sheet with viscous dissipation. The governing partial differential equations are transformed into nonlinear coupled ordinary differential equations with the help of suitable similarity transformations. These equations were then solved numerically by using an implicit finite difference method known as Keller-Box method. The effects of various parameters such as magnetic parameter (M), Casson parameter (β), local Grashoff number (Gr), local modified Grashoff number (Gc), nonlinear parameter (n), Eckert number (Ec) on velocity, temperature and concentration were discussed and presented graphically. It is found that a larger value of Casson parameter leads to decrease the velocity and temperature. Increase in the local Grashoff number reduces the temperature. Nanoparticle concentration is decreased for the larger values of local Modified Grashoff number. The numerical values of skin friction, Nusselt number and Sherwood number are presented in tables.