This paper focuses on the understanding of transitional and self-similar mixing layers which undergo rapid anisotropic strain. Anisotropic strain is present in a multitude of practical applications, for example in any circumstance where a mixing layer passes through compression or expansion regions caused by geometry (convergence or expansion) or the presence of shock or rarefaction waves. The impact of anisotropic strain is often modelled either through the assumption of isotropic compression or expansion based on the mass-weighted velocity (Dimonte and Tipton 2006) or models which use the strain rate in the principal direction of mixing (Youngs,1994). However, the fidelity of these model assumptions have not yet been examined for cases where the strain rates or flow composition is significantly anisotropic, such as occurs in a planar mixing layer between two fluids of differing densities. This research uses a novel numerical setup which enables the isolation of strain effects from other potential drivers of turbulent mixing such as buoyancy driven mixing layer growth or suppression (the Rayleigh-Taylor instability). This is achieved through (i) the super-position of a mean strain rate on top of the flow field and (ii) the derivation of an appropriate momentum source term and boundary conditions which remove the pressure gradient in a similar manner to that used in periodic channel flows. This computational setup is employed to examine the evolution of a turbulent mixing layer generated from the Richtmyer-Meshkov instability exposed to a mean strain rate using Implicit Large-Eddy Simulation. It provides detailed flow field data needed to advance current engineering model formulations. We present both an analysis of the influence of strain on key properties of the mixing layer with a particular focus on mixing width, kinetic energy and mixedness, and propose improved Buoyancy-Drag and URANS models. G. Dimonte and R. Tipton. K-L turbulence model for the self-similar growth of the Rayleigh-Taylor and Richtmyer-Meshkov instabilities. Phys. Fluids, 18:085101, 2006. D. L. Youngs. Numerical simulation of mixing by Rayleigh-Taylor and Richtmyer- Meshkov instabilities. Laser Part. Beams, 12:725–750, 1994.