A Mainframe Developer specializes in designing, developing, and maintaining applications on mainframe systems, crucial for organizations reliant on legacy technologies. Responsibilities include coding, testing, and debugging, often utilizing languages like COBOL and DB2. Required skills include problem-solving and proficiency in mainframe tools. Typically working in IT departments, collaboration occurs with system analysts and project managers. Career progression may lead to senior developer or architect roles, with opportunities to transition into modern technologies or IT leadership.