Lower-extremity orthosis is a type of wearable mechanical device that serves a wide variety of important biomedical purposes, such as gait assistance and rehabilitative training. Due primarily to the constraints associated with actuation technology, the majority of current lower-extremity orthoses are either passive or tethered to external power sources, limiting the functionality of such devices. In this paper, the authors present the research results towards a fully mobile (i.e., untethered) powered lower-limb orthosis, leveraging the high power density of pneumatic actuators for the joint power generation. The design of the orthosis is presented, with the objectives of providing full locomotive assistance in multiple common locomotive modes and generating a minimum level of restriction to the wearer's daily activities. For the control of the orthosis, a finite-state impedance-based controller is developed, which simulates an artificial impedance in order to enable the natural interaction with the wearer. Preliminary testing on a healthy subject demonstrated that the orthosis was able to provide a natural gait and a comfortable user experience in the treadmill walking experiments.