This paper reports the development of a robust controller based on backstepping sliding mode control (SMC) for a four-wheel-steering (4WS) and four-wheel-drive (4WD) agricultural robotic vehicle. Two RTK-GPS receivers were installed on the vehicle to provide feedback of the robot’s location and orientation. Kinematic models of front wheel steering and coordinated steering modes and their corresponding navigation controllers were developed. Diverse reference trajectories were designed and used to validate the controller performance. The experimental results demonstrated the capability and robustness of the developed SMC navigation controller in controlling a nonholonomic system that has a high degree of freedom.