The influence of linear (spanwise-uniform) forcing applied by a DBD plasma actuator on the growth of a turbulent mixing layer and the dynamics of large-scale spanwise vortices, are investigated experimentally. Furthermore, the freestream turbulence intensity in the low-velocity stream is maintained at a high level to examine its impact on the control-authority of the applied forcing. The influence of the applied forcing on the dynamics and interactions of the spanwise vortices in the mixing layer is apparent. However, the growth rate of the mixing layer is not affected, suggesting that the high level of freestream turbulence diminishes the control-authority of the applied forcing.