Representing upper ocean turbulence accurately in models remains a great challenge for improving weather and climate projections. Langmuir circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. We present a direct comparison between observations and large eddy simulations, based on the wave-averaged Navier-Stokes equation, of an LC growth event. The evolution of cross-wind velocity variance and spatial scales, as well as mixed layer deepening are only consistent with simulations if LC effects are included in the model. Our results offer a validation of the large eddy simulation approach to understanding LC dynamics, and demonstrate the importance of LC in ocean surface layer mixing.