A MERN developer specializes in the MERN stack - MongoDB, Express.js, React.js, and Node.js - for web application development. They design and implement user-facing features using React.js, build back-end services with Node.js and Express.js, and manage data storage with MongoDB. They develop responsive, efficient, and scalable applications, ensuring both client and server-side performance optimization. They also participate in code reviews, testing, and debugging, and often collaborate with other team members and stakeholders. Their role involves understanding business requirements, translating them into technical specifications, and constantly updating their skills with emerging technologies.