Mobile Learning Games (MLGs) show great potential for increasing engagement, creativity and authentic learning. Yet, despite their great potential for education, the use of MLGs by teachers, remains limited. This is partly due to the fact that MLGs are often designed to match a specific learning context, and thus cannot be directly reusable for other contexts. Researchers have therefore designed various types of MLG authoring tools. However, a recent study we led shows that these authoring tools are ill-adapted to the teacher’s needs and competencies. The teachers find them either too poor to create MLGs that fit their teaching requirements, or too complex and overwhelming to use. In this paper, we introduce JEM iNVENTOR, an authoring tool based on a nested design approach. It offers three conceptual levels adapted to the teacher’s competencies, and allows them to progress in time.