Precision sensing, and in particular high precision magnetometry, is a central goal of research into quantum technologies. For magnetometers, often trade-offs exist between sensitivity, spatial resolution, and frequency range. The precision, and thus the sensitivity of magnetometry, scales as $1/sqrt {T_2}$ with the phase coherence time, $T_2$, of the sensing system playing the role of a key determinant. Adapting a dynamical decoupling scheme that allows for extending $T_2$ by orders of magnitude and merging it with a magnetic sensing protocol, we achieve a measurement sensitivity even for high frequency fields close to the standard quantum limit. Using a single atomic ion as a sensor, we experimentally attain a sensitivity of $4.6$ pT $/sqrt{Hz}$ for an alternating-current magnetic field near 14 MHz. Based on the principle demonstrated here, this unprecedented sensitivity combined with spatial resolution in the nanometer range and tunability from direct-current to the gigahertz range could be used for magnetic imaging in as of yet inaccessible parameter regimes.
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