A Mercurial developer is responsible for using the Mercurial Distributed Version Control System (DVCS) to manage and track changes to project source code. They handle code revisions, merge conflicts, and versions, ensuring seamless collaboration among developers. They commit changes, create repositories, and keep historical records of every modification. They also use Mercurial's features like branching and merging to develop new features without affecting the main codebase. Their role involves improving code quality, facilitating code reviews, and ensuring smooth software development workflow. Furthermore, they might also be involved in setting up and maintaining the Mercurial infrastructure.