To address the issues of environmental interference and high cost of force sensors in electro-mechanical brake (EMB) systems, this paper proposes a sensorless clamping force estimation method and a composite control strategy. An electromechanical model of the EMB system is established and an improved sliding mode observer using a saturation function is designed to reduce chattering. The finite-time convergence of the observer is proven via Lyapunov theory. An extended state observer is also designed to estimate and compensate for unmodeled disturbances via feedforward control. Furthermore, a novel reaching law is developed by modifying the dual-power reaching law with a variable-gain function to suppress chattering while maintaining convergence speed. A controller based on a nonsingular terminal sliding mode and the improved reaching law is constructed. Bench tests show that the proposed method outperforms dual-power, exponential reaching law and PID methods, offering faster force tracking, higher precision, and enhanced anti-interference capability under various braking conditions.