A new nonlinear switching surface is proposed for the design of a sliding mode controller for a class of mismatched uncertain systems. A control system for slosh-free motion of a partially filled liquid container is sought. This is representative of a broader class of systems to which the proposed methodology is applicable. A fundamental mode of lateral slosh is considered. A simple pendulum model is used to represent the lateral slosh. Given the difficulty of measuring the slosh states directly, a sliding mode observer is used for implementation of the resulting sliding mode control. The effectiveness of the theoretical developments is demonstrated via both simulation and experimental results.