A laser-induced fluorescence (LIF) technique is implemented to measure concentration profiles and mixing performance in two impinging jet geometries. Experimental results are reported for steady and unsteady (transitional) mixing of initially segregated miscible fluids. Flow structures are visualized by imaging concentration distributions at five vertical planes throughout the mixers. Mixing is quantified for each Reynolds number examined by calculating the overall intensity of segregation. Mixing performance varies substantially as a function of Reynolds number. Optimum operating conditions are identified for both impinging jet geometries. The results demonstrate the ability of laser-induced fluorescence to quantitatively capture small- and large-scale flow structures and accurately and reproducibly quantify mixing performance in real time for industrially-relevant mixing devices. The LIF technique proves to be an accurate and versatile method to quantify mixing performance of miscible fluids.