Understanding and mitigating decoherence is a key challenge for quantum science and technology. The main source of decoherence for solid-state spin systems such as quantum dots, donors in silicon and defects in diamond is the uncontrolled spin bath environment. Here, we demonstrate quantum control of a mesoscopic electron spin bath in diamond at room temperature. The resulting spin bath dynamics are probed using a single nitrogen-vacancy (NV) centre electron spin as a magnetic field sensor. We exploit the spin bath control to dynamically suppress dephasing of the NV spin by the spin bath. Furthermore, using ideas from dynamical decoupling, we directly measure the coherence and temporal correlations of different groups of bath spins. These results uncover a new arena for decoherence studies and may provide novel avenues for protecting the coherence of solid-state spin qubits. Moreover, spin bath control is a crucial ingredient of recent proposals for environment-assisted magnetometry, room-temperature quantum computing using spins in diamond and spin squeezing.