Organic semiconductors that emit by the process of multi-resonance thermally activated delayed fluorescence (MR-TADF) can deliver narrowband and efficient electroluminescence while being processable from solvents and metal-free. This renders them attractive for use as the emitter in sustainable light-emitting electrochemical cells (LECs), but so far reports of narrowband and efficient MR-TADF emission from LEC devices are absent. Here, this issue is addressed through careful and systematic material selection and device development. Specifically, the authors show that the detrimental aggregation tendency of an archetypal rigid and planar carbazole-based MR-TADF emitter can be inhibited by its dispersion into a compatible carbazole-based blend host and an ionic-liquid electrolyte, and it is further demonstrated that the tuning of this active material results in a desired balanced p- and n-type electrochemical doping, a high solid-state photoluminescence quantum yield of 91%, and singlet and triplet trapping on the MR-TADF guest emitter. The introduction of this designed metal-free active MR-TADF material into a LEC, employing air-stabile electrodes, results in bright blue electroluminescence of 500 cd m−2, which is delivered at a high external quantum efficiency of 3.8% and shows a narrow emission profile with a full-width-at-half-maximum of 31 nm.