Scalar composition and mixing rates within a non-reactive plane shear layer between two uniform water streams were manipulated by surface actuators on the high-speed side of the flow partition. The scalar concentrations were assessed using a thermal analogue by maintaining a time-invariant temperature difference between the uniform streams upstream of the flow partition. The ratio of the smallest velocity and temperature scales was governed by the Prandtl number. Because in water $\hbox{\it Pr}\,{ \approx}\,7$ while the Schmidt number $\hbox{\it Sc}$ for dyes and reactants is O(1000), temperature concentrations in water are a better representation of scalar mixing in air in which $\hbox{\it Sc}\,{=}\,O(1)$. The effects of spanwise-uniform and -non-uniform actuation programs were investigated using arrays of discrete, individually controlled thin-film resistive heating elements that were surface-mounted on the flow partition. Spatial and temporal temperature distributions were measured phase-locked to the actuation waveform using a cross-stream array of closely spaced cold-wire sensors. These data were used to infer both the mixedness and the composition through the onset of mixing transition and quantify local and integral cross-stream mixing performance measures. Actuation programs that hastened mixing and significantly altered the composition of mixed fluid were identified.