A model for the growth of a convectively mixed layer is derived by layer integrating the basic equations and parameterizing unknown terms in the mixed layer turbulence kinetic energy equation by means of free convection similarity theory. When shear generation of turbulence energy is neglected in the turbulent inversion layer capping the mixed layer, the model essentially reduces to that of Tennekes. This shear generation is found to be important only in cases of significant baroclinicity and shallow mixed layer depth or small free flow stratification.