Mixed reality (MR) technology combines digital elements with the real world, offering immersive experiences in various fields. However, content creation in MR remains challenging for average users due to complex 3D modelling techniques. This paper introduces Light-Tracing, a novel system integrating light painting with MR environments using a robotic arm. Light-Tracing allows users to intuitively create 3D models through path light painting, which AI tools convert into digital assets. The system simplifies spatial content input, making MR content creation more accessible and enhancing user experience. Evaluations demonstrate its positive impact on usability and immersion. Importantly, Light-Tracing lowers the barrier for MR content creation and opens new possibilities for participatory design in urban planning and vehicle customization, highlighting its potential to revolutionize MR content creation.