Autonomous mobile robots are being developed for numerous applications where long-term capabilities would be beneficial. However, most mobile robots have on-board power supplies in the form of batteries that last for a finite amount of time, in which case the robot becomes reliant on human intervention for extended usage. To achieve true long-term autonomy, the robot must be self-sustaining in its environment. We have developed a control architecture and an accompanying recharging mechanism which allows a robot to readily intervene its regular operation with autonomous recharging to stay alive. We demonstrate the efficacy of our system experimentally, by requiring the robot to serve as a sentry, monitoring our lab entrance for an extended period of time. The system is able to operate for long periods of time without operator intervention.