In this paper, a class of interconnected systems with mismatched uncertainties is considered. A sliding surface is proposed in an augmented space formed by the system output and the compensator state variables, and the stability of the corresponding sliding mode is analysed. A robust decentralised reduced-order output feedback sliding mode controller is then designed to drive the system to the composite sliding surface and maintain a sliding motion on it thereafter. The proposed approach allows both the isolated nominal subsystem and the nominal interconnected system to be nonminimum phase. The known interconnections and the bounds on the uncertain interconnections are fully used in the control design to reduce conservatism.