Distraction osteogenesis is a technique of bone lengthening that makes use of the body’s natural healing capacity. An osteotomy is created, and a rigid distraction device is attached to the bone. After a latency period, the device is activated two to four times per day for a total of 1 mm/day of bone lengthening. This technique is used to correct a variety of congenital and acquired deformities of the mandible, midface, and long bones. To shorten the treatment period and to eliminate the complications of patient activation of the device, an automated continuous distraction device would be desirable. It has been reported that continuous distraction generates adequate bone with lengthening at a rate of 2 mm/day, thereby reducing the treatment time. The device we describe here uses miniature high-pressure hydraulics, position feedback, and a digital controller to achieve closed-loop control of the distraction process. The implanted actuator can produce up to 40 N of distraction force on linear trajectories as well as curved distraction paths. In this paper we detail the spring-powered hydraulic reservoir, controller, and user interface. Experiments to test the new device design were performed in a porcine cadaver head and in live pigs. In the cadaver head, the device performed an 11 day/11 mm distraction with a root-mean-squared position error of 0.09 mm. The device functioned for periods of several days in each of five live animals, though some component failures occurred, leading to design revisions. The test series showed that the novel design of this system provides the capabilities necessary to automate distraction of the mandible. Further developments will focus on making the implanted position sensor more robust and then on carrying out clinical trials.