Transition metal dichalcogenide (TMDC) monolayers are promising materials for next-generation nanoscale optoelectronics, including high-speed light sources and detectors. However, most past reports on TMDC light-emitting diodes are limited to operation in high vacuum, while most applications require operation under ambient conditions. In this work, we study the time-resolved electroluminescence of monolayer WSe2 p-n junctions under ambient conditions and identify the decay in current over time as the main issue preventing stable device operation. We show that pulsed voltage bias overcomes this issue and results in bright electroluminescence under ambient conditions. This is achieved in a simple single-gate structure, without the use of dual gates, heterostructures, or doping methods. Internal quantum efficiency of electroluminescence reaches similar to 1%, close to the photoluminescence quantum efficiency, indicating efficient exciton formation with injected carriers. Emission intensity is stable over hours of device operation. Finally, our device exhibits similar to 15ns rise and fall times, the fastest direct modulation speed reported for TMDC light-emitting diodes. Published under license by AIP Publishing.