An AutoMapper developer utilizes the AutoMapper library in .NET projects to simplify object mapping. They use it to eliminate the need for manually writing code to convert one object type into another. This is crucial in layered architectures where data must be transferred between objects of different types, such as between domain and view models. AutoMapper developers are responsible for configuring mappings between object types, handling complex scenarios like nested mappings, and ensuring efficient data transformation. They also conduct testing to ensure the accuracy of mappings and troubleshoot any issues that arise during the object transformation process.