In stratified natural waters, convective processes tend to form nearly homogeneous mixed layers. However, shear‐driven turbulence generated by large‐scale background flow often rapidly smooths them through mixing with the stratified surroundings. Here we studied the effect of background turbulence on convectively driven mixed layers for the case of bioconvection in Lake Cadagno, Switzerland. Along with microstructure measurements, a diffusive‐shape model for the mixed layers allowed us to define (i) mixed layer thickness and (ii) diffusive transition length. Further microstructure analysis was performed allowing estimation of convective turbulence in the mixed layer and shear‐driven turbulence quantified by eddy diffusion in their surroundings. Based upon these results, we propose a Péclet number scaling that relates mixed layer shape to the opposing effects of convection and diffusion. We further validate this quantitative approach by applying it to two other distinct convective systems representative of double‐diffusive convection and radiatively driven under‐ice convection.