A Mantis developer works on the Mantis Bug Tracker, an open-source web-based bug tracking system. They are responsible for developing and maintaining the system, which involves writing code, fixing bugs, and making improvements to its functionality. They also collaborate with other developers, participate in code reviews, and provide technical support to users. They ensure the system is user-friendly, stable, and secure. Moreover, they implement new features based on user feedback, and update the system to keep up with technological advancements. Furthermore, they may also create documentation to guide users and other developers.