This work focused on the study of Soret and Dufour effects on unsteady coupled heat and mass transfer by mixed convection flow over a vertical cone rotating in an ambient fluid with a time-dependent angular velocity in the presence of a magnetic field and chemical reaction. The cone surface is maintained at variable temperature and concentration. The resulting governing equations are non-dimensionalised and transformed into a non-similar form and then solved numerically by an implicit, iterative, finite-difference method. Comparisons with previously published work are performed and excellent agreement is obtained. A parametric study showing the effects of the buoyancy parameter, magnetic field, chemical reaction parameter, Soret and Dufour numbers on the local tangential and azimuthal skin friction coefficients, and the local Nusselt and Sherwood numbers are conducted. These results are illustrated graphically to depict special features of the solutions.