We demonstrate $4\pi$-steradian vector magnetic field sensing using an ensemble of nitrogen-vacancy ($\mathrm{N}$-$V$) centers in a single-crystal diamond coupled to a microwave (MW) cavity. The MW cavity enhances the spin-photon coupling, which enables efficient, high-contrast spin-state readout via MW interrogation and removes the need for bulky optical collection components. A slowly varying sinusoidal bias magnetic field lifts the zero-field degeneracy of the four crystallographic $\mathrm{N}$-$V$ orientations, allowing each orientation to be individually addressed and used for vector reconstruction of the magnetic field. The resulting magnetometer has a 36% contrast (20 times higher than typical for optical spin-ensemble readout) and achieves a single-axis sensitivity of 250 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$ that is flat from direct current to 1 kHz. Noise models of the composite spin-cavity system establish MW amplitude noise as the dominant noise source and predict a thermal-noise limit of 2 $\mathrm{pT}/\sqrt{\mathrm{Hz}}$.

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• Magnetometry
• Quantum control
• Quantum sensing
• Nitrogen vacancy centers in diamond
• Microwave techniques