Cam–follower kinematic pairs in the valve trains of engines usually operate in the mixed lubrication regime and work under conditions of wear. The wear mode of cams in mixed lubricated contacts is mild, and the wear value is usually calculated using Archard’s equation in which the wear coefficient is closely related to the lubrication condition. In this study, a nonlinear relationship between the wear coefficient and lubricant oil film thickness under mixed lubrication is proposed with consideration of the rough height distribution of the contact surface. As the cam and flat tappet pair of the valve train in a diesel engine is concerned, the wear coefficient of the cam material for boundary lubrication conditions is measured. After calculating the lubricant oil thickness, the wear depths at different points of the cam are simulated, and results are consistent with the measured values in the practical test.