We employed time-resolved, in situ neutron reflectometry to observe a dynamic electrode–electrolyte interface under conditions relevant to Li-mediated electrochemical N2 reduction reaction (NRR). This method leverages the sensitivity of neutrons to Li and fast time resolution (∼1 min) to observe the formation of a layer containing Li species at the electrode surface within minutes of applying a current density (−0.1 mA/cm2). Notably, within the first 6 min, we did not observe a solid-electrolyte interphase (SEI) distinct from this layer, providing insight into recent reports demonstrating performance advantages of short current cycles interspersed with open-circuit conditions for NRR. At longer time scales following chronopotentiometry (∼2 h), a multilayer SEI remained, though the presence of Li was not evident, indicating that the layers containing Li species observed over shorter time scales degrade almost entirely under open-circuit conditions, leaving SEI layers consisting of electrolyte decomposition products. We thus present the first application of neutron reflectometry toward the NRR─through fast time-resolved measurements, we have enabled a path toward understanding the electrochemical NRR as well as dynamic systems across a wide range of energy technologies.