The paper proposes a nonlinear full-state feedback controller to overcome model uncertainty as well as external disturbances in electrohydraulic servo systems. The controller combines sliding mode control and a finite time disturbance observer through the backstepping method to achieve effective control performance. First, the finite time observer estimates the full state signal, including the position signal, velocity signal, acceleration signal, and the lumped disturbance in the electro-hydraulic systems. Furthermore, a composite reaching law using a novel hyperbolic tangent function, along with sliding mode control, adaptively adjusts the convergence rate of the equilibrium point while effectively improving the chattering problem. Using Lyapunov theory, it is proven that the proposed controller achieves asymptotic convergence, while the observer ensures finite-time convergence. Finally, in order to prove the performance of the designed control strategy, simulation results are presented.