This paper presents a Model Reference Adaptive Controller (MRAC) for the active steering of a trailer to improve the lateral stability of Car-Trailer (CT) systems. A 3 Degrees-of-Freedom (DOF) linear yaw-plane CT model is developed as the Reference Model. The yaw rate of the leading and the trailing units of the reference model are tracked to control and stabilize a non-linear CT model. Lyapunov-stability-based MRAC technique is employed to handle the dynamics of the CT system. The model parameters and operating conditions of the vehicle system are assumed to be constant while designing the controller. The effectiveness of the MRAC controller is demonstrated using numerical simulations of the non-lienar CT model under a single lane-change maneuver.