The influence of spanwise-uniform electro-fluid-dynamic forcing applied by a dielectric barrier discharge (DBD) plasma actuator on the growth of a plane mixing layer and the dynamics of large-scale spanwise vortices are investigated experimentally. A two-dimensional mixing layer formed between two streams of air with different velocities is employed for this study. Quantitative spatio-temporal measurement of the flow field is acquired using high-speed planar particle image velocimetry. The DBD actuator was constructed such that it imparts perturbations into the splitter-plate boundary layer formed by the high-velocity fluid stream, close to the trailing edge. Through this, the fundamental Kelvin-Helmholtz instability of the current mixing layer and its first sub-harmonic are forced. Forcing the fundamental instability results in the inhibition of vortex pairing due to the attenuation of sub-harmonic instabilities, and thus, mixing layer growth is halted in the vicinity of the trailing edge. With sub-harmonic instability forcing, neighboring vortices interact with each other and amalgamate together through mutual induction. This results in a higher growth rate compared to the unforced mixing layer at the streamwise location of this vortex interaction. Eventually, the growth rate of the forced mixing layers becomes similar to that of the unforced case. These results demonstrate the influence of the applied forcing on the spectral signature, growth, and stability characteristics of the plane mixing layer and the dynamics of the coherent vortical structures.