A MongoDB developer designs and manages databases using MongoDB, a NoSQL database program. They handle data modeling, create schemas, and perform CRUD operations. They ensure the performance, availability, and scalability of the database system. The developer handles data migration and transformation services and manages the database service configurations. They also develop and maintain business logic in the application layer, integrating it with MongoDB. They work on optimization, security, and data validation. Understanding of MongoDB's sharding and replication features is also crucial. They troubleshoot issues, provide solutions, and often collaborate with other team members.