This work presents a control methodology that enables nonlinear model based sliding mode control of PWM-controlled pneumatic servo actuators. An averaging approach is developed to describe the equivalent continuous-time dynamics of a PWM controlled nonlinear system, which renders the originally discontinuous system, possibly non-affine in the input, into an equivalent system that is in control canonical nonlinear form. A sliding mode controller is then developed for a pneumatic servo actuator based on the averaged equivalent control canonical model. The controller is implemented on an experimental system, and the effectiveness of the proposed approach validated by experimental tracking data.