Abstract Large eddy simulation (LES) of a compressible mixing layer is carried out to investigate scalar mixing in mixed supersonic-subsonic shear layers. The ability of two sub-grid models to predict scalar mixing in compressible shear layers is assessed in this study. The first one employs a conventional gradient diffusion model whereas the second one uses the linear eddy mixing (LEM) model. Comparison of the results using these two models with experimental data shows that the LEM sub-grid closure is capable of accurately capturing the effect of scalar mixing in supersonic shear layers within the framework of LES. In particular, mean and root mean square of the mixture fraction are in excellent agreement with data whereas the gradient closure is unable to capture the observed physics. These studies have established a new LES method that is capable of predicting the scalar mixing in compressible shear layers.