The optical and material properties of negatively charged nitrogen–vacancy (NV) centres in diamond make them attractive for applications ranging from quantum information to electromagnetic sensing. These properties are strongly dependent on the vibrational manifold associated with the centre, which determines phenomena associated with decoherence, relaxation and spin–orbit coupling. Despite its paramount importance in tuning these properties, the role of the vibrational bath and its effect on the electronic-state dynamics of NV centres in diamond is not fully understood. To elucidate the role of the bath, we present two-dimensional electronic spectroscopic studies of ensembles of negatively charged NV defect centres in diamond (NVD). We observe picosecond non-radiative relaxation within the phonon sideband and find that strongly coupled local modes dominate the vibrational bath. These findings provide a starting point for new insights into dephasing, spin addressing and relaxation in NVD with broad implications for magnetometry, quantum information, nanophotonics, sensing and ultrafast spectroscopy.