High-sensitivity magnetometry has found important applications in fields ranging from basic science studies such as searching for dark matter and exotic particles to more practical tasks in geology, archaeology, navigation, and biomedicine. Currently, the performance of typical high-sensitivity magnetometers is limited by the required stringent operation environment, such as cryogenic conditions for superconducting quantum interference device magnetometers and near-zero-field environments for spin-exchange-relaxation-free atomic magnetometers. This Letter reports a high-sensitivity solid-state magnetometer based on a magnetostrictive gap-swing Fabry-Pérot cavity optomechanical system that is capable of benchmark performance at ambient environment conditions. Thanks to the strong resonance enhancement of the gap-swing mechanical mode, it achieves a sensitivity of $620\mathrm{fT}{\mathrm{Hz}}^{-1/2}$ at room temperature and under the Earth’s magnetic field, and is expected to approach the thermal-noise-limited sensitivity of $5.9\mathrm{fT}{\mathrm{Hz}}^{-1/2}$ by controlling the optomechanical coupling. Our Letter opens the avenue toward the application of portable and low-maintenance high-sensitivity magnetometry in broad fields.