A MithrilJS developer designs and builds web applications using the Mithril.js framework, a modern, lightweight, and robust tool for creating single-page applications. They use its small API to create high-performance applications, focusing on rendering views and managing state. They also handle routing and XHR utilities, making use of Mithril's hyperscript and virtual DOM diffing. They work with other technologies like HTML, CSS, and JavaScript, ensuring smooth integration. MithrilJS developers also write tests, troubleshoot, and maintain the software, continually improving its performance and user experience. They collaborate with teams, sharing their expertise and implementing best practices.