A Mercury Quality Center developer is responsible for creating, implementing, and managing testing processes using Mercury Quality Center (now known as HP ALM). They design test plans, develop manual and automated test scripts, and execute these to validate software functionality and performance. Their role also involves tracking and documenting defects found during testing, collaborating with the development team to resolve them, and ensuring overall software quality. They may also provide training to other team members on the use of Quality Center and contribute to the continuous improvement of testing methodologies.