The onset of mixing in the shear layer located between two coincident laminar streams of different velocity in a thin, ribbon-like channel is studied. Globally unstable density stratification is employed, which, with certain experimental conditions, results in augmented mixing and entrainment, and disruption of mass transfer of solid particles compared to channels with stable density gradients, and laminar, two-dimensional flow. The mixing onset boundary is quantified in terms of the Reynolds number, Richardson number, and channel flow rate ratios using a newly developed mixing onset parameter R. The results indicate that unstable density gradients, rather than shear gradients, are most responsible for inducing the mixing.