Modern passive ankle-foot prostheses do not exhibit appropriate biomechanics during walking, and are unable to adjust their mechanics for other mobility tasks, such as stair traversal or quiet standing. In this paper, we introduce a quasi-passive ankle-foot prosthesis that addresses these challenges; the ankle has a customizable, nonlinear torque-angle curve, and the overall stiffness can be varied continuously between mobility tasks. The variation in mechanics is accomplished by integrating two mechanisms: a cam-based transmission, in which rotation of the ankle joint causes deflection of a leaf spring, and an active sliding support beneath the leaf spring, which can modify the spring's effective stiffness. In addition to introducing the design, we present the mathematics to calculate the cam profile for any arbitrary torque-angle curve, and experimentally characterize the system for a desired curve based on human walking. Lastly, we demonstrate the full range of stiffness levels available and stiffness transition time.