A Mithril.js developer designs and implements user interface components for JavaScript-based web and mobile applications using the Mithril.js framework. They focus on the efficiency, reliability, and speed of the applications. Their tasks include creating modules and components, connecting them to create a functioning app, and writing reusable, testable, and efficient code. They also ensure high performance of applications, troubleshoot and resolve issues, and collaborate with other team members. Furthermore, they ensure the technical feasibility of UI/UX designs and optimize applications for maximum speed and scalability. They may also conduct code reviews and maintain documentation.