Immersive reality (XR) technologies, particularly Mixed Reality (MR), offer promising opportunities for enhancing design prototyping. While recent studies often focus on Virtual Reality this work explores the application of MR, where focus lies on interlinking both the physical and digital to maximise benefit. Following a review of XR in design, a descriptive framework is presented to characterise MR prototyping. Two case studies are then presented to highlight the value of bridging the physical and digitalf worlds, before directions for further research in MR-based prototyping are outlined.