The present analysis deals with the numerical investigation of steady laminar quadratic convective flow of a viscous incompressible viscoplastic Casson fluid external to a circular vertical cone under radial magnetic field and non-uniform surface temperature. The problem is governed by a system of coupled nonlinear boundary layer equations with appropriate variable surface boundary conditions. These governing equations are reduced to a non-dimensional form and are solved numerically using implicit finite difference method, which is unconditionally stable and convergent. The influence of physical parameters such as nonlinear temperature (α), magnetic parameter (M) Casson fluid parameter (β), thermal radiation parameter (F) and suction parameter (fw) on velocity profiles and temperature profiles has been shown graphically. Validation of solutions with earlier published work is included. The computations show that the flow near the cone surface is strongly accelerated with increasing nonlinear temperature whereas the temperature and thermal boundary layer thickness are decreased.