A Mapnik developer creates and manages visual representations of geospatial data. This involves designing maps by defining their aesthetics, such as colors, patterns, and styles, using Mapnik's cartographic language. They integrate data from various sources, including GIS databases and web services, to create dynamic, interactive maps. They also work on improving the performance and functionality of Mapnik's rendering engine, ensuring that it can handle large datasets and complex visualizations efficiently. Additionally, they may contribute to the open-source Mapnik project, fixing bugs, adding new features, and helping to maintain the software's documentation and user community.