Drones are increasingly being used for a variety of applications, but their use can be hampered when Global Navigation Satellite Systems (GNSS) signals are unavailable. This paper presents Terrain Referenced Navigation (TRN) from a class 1 drone using low-cost, automotive-class mm-wave radar to enable accurate navigation in GNSS-denied environments. The radar-derived elevation profile of the ground beneath the drone as it flies is compared to a lidar-based digital terrain map and, using a particle filter, the drone’s position is estimated. Results show that mm-wave radar TRN can provide an accurate navigation capability in the absence of GNSS and offers several advantages over other alternative navigation technologies.