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 strongcoupled characteristics, model uncertainties and external disturbance. Finally, simulation results under different boundary constraints further validate the feasibility and good robustness of the controller.