In this paper, a novel motion control scheme with guaranteed prescribed performance is proposed for autonomous vehicles with consideration of couplings between lateral and longitudinal motions. Firstly, a prescribed performance transformation function is constructed to equivalently release the output error constraints. Then, a coordinated controller is designed to complete lateral and longitudinal motion control tasks simultaneously based on sliding-mode control. The designed coordinated controller can guarantee predefined trajectory tracking performance (e.g., minimum speed of convergence, maximum steady state error and overshoot), in presence of strong-coupled characteristics, model uncertainties and external disturbance. Finally, simulation results under different boundary constraints further validate the feasibility and good robustness of the controller.