Abstract We applied an integrated UAV‐ and ground‐based magnetometry workflow to map shallow subsurface fractures and cavities in the town of Grindavík during the 2023–2025 volcano‐tectonic crisis. Short‐wavelength linear and semi‐circular magnetic lows delineate open or partially open fractures and localized cavities, including features with no surface expression, verified through field observations, LiDAR surface deformation, and targeted excavations. Several anomalies correspond to reactivated fractures identified through historical aerial imagery. Magnetic forward modeling shows anomaly shapes and amplitudes compatible with fracture and void sources within the upper ∼10–20 m. The approach enabled rapid mapping of major fracture networks in inaccessible terrain, revealing fracture continuity, aperture variability, and branching patterns that guided hazard assessment and emergency response. This study demonstrates the first combined UAV‐ and ground‐based magnetometry application for near real‐time fracture mapping during an urban volcano‐tectonic crisis with implications for rapid hazard response in other tectonically active settings with sufficient magnetic contrast.