A Mercurial QT developer is responsible for creating and managing software applications using Mercurial, a distributed version control system, and QT, a cross-platform software development framework. They write, debug, and optimize code, ensuring its compatibility across different platforms and devices. They also use Mercurial to track changes, manage versions, and facilitate collaboration between team members. Their work involves creating user-friendly interfaces with QT, integrating databases, and maintaining the security and performance of applications. Additionally, they troubleshoot issues, implement new features based on user feedback, and stay updated with the latest industry trends and technologies.