The magnetohydrodynamic problem of a fully developed flow of an incompressible and electrically conducting fluid is solved numerically by a Chebyshev spectral collocation method in a square duct with walls of variable electric conductivities under a slip condition for velocity. The flow is driven by a constant pressure gradient under the effect of an externally applied oblique magnetic field. The efficiency of the method that is implemented in the physical space on preassigned collocation points is exploited to discretise the governing equations. The corroboration and validation of the proposed technique are carried out by means of a case study with published results substantiating that its implementation results in satisfactorily good agreements. Novel results are presented graphically, and the combined effects of the most characteristic magnetohydrodynamic flow parameters such as the slip length, conductivity parameter, and Hartmann number on the velocity and induced magnetic field are investigated.