Ocean fronts play a key role in the vertical transport of heat, energy and biogeochemical tracers, but their behaviour and life cycle is not well-understood, especially at smaller scales. We present a large-eddy simulation of mixed layer frontogenesis capturing the onset of a submesoscale regime from a larger scale, rotation-dominated front. Frontogenesis shifts the balance between vertical mixing and restratification driven by frontal ageostrophic secondary circulation and the final state is a combination of a small, turbulent arrested region and the restratified remainder of the front, which sits above an unmixed, stable region. The transition to this state is marked by the appearance of down-front symmetric cells, which coalesce to form a circulation that entrains fluid from the mixed layer base. We describe the results in the context of submesoscale instability and potential vorticity transport and discuss the implications for frontogenesis and frontal arrest in the ocean mixed layer.