In this article, the heat, momentum and mass (species) transfer in external boundary layer flow of Casson nanofluid from an isothermal sphere surface is studied theoretically. The effects of Brownian motion and thermophoresis are incorporated in the model in the presence of both heat and nanoparticle mass transfer. The governing partial differential equations (PDEs) are transformed into highly nonlinear, coupled, multi-degree non-similar partial differential equations consisting of the momentum, energy and concentration equations via appropriate non-similarity transformations. These transformed conservation equations are solved subject to appropriate boundary conditions with a second order accurate finite difference method of the implicit type. The influences of the emerging parameters i.e. Casson fluid parameter (β), Buoyancy ratio parameter (N), Brownian motion parameter (Nb) and thermophoresis parameter (Nt), Lewis number (Le) and Prandtl number (Pr) on velocity, temperature and nano-particle concentration distributions is illustrated graphically and interpreted at length. Validation of solutions with a Nakamura tridiagonal method has been included.