A theory of the non-linear longitudinal Kerr magneto-optic effect, previously developed for bulk materials [1], is now applied to a magnetic thin film. The material model is based on the classical equation of motion for a free electron with a finite relaxation frequency under the action of a Lorentz force. A second harmonic current density, dependent upon three non-linear conductivity tensors, reduces to results obtained by Jha [2] in the limit of vanishing ferromagnetic state. The second harmonic reflection coefficients are approximated and are given in terms of the first harmonic reflection coefficients as derived by Smith [3].