A Leiningen developer works with the Leiningen tool, primarily used in the Clojure programming language. They manage project definitions and dependencies, automate tasks using scripts, and handle the project's lifecycle. Their responsibilities include writing Clojure code, building software packages, running tests, and deploying applications. They utilize Leiningen's features for handling classpath, running a REPL (Read-Eval-Print Loop) for interactive development, or generating standalone JAR files. They also use it to manage libraries and assist in the creation of new projects. The developer's goal is to streamline and simplify the development process using Leiningen.