Ionospheric F‐region electron density is anomalously higher in the evening than during the daytime on many occasions in the summer in geomagnetic mid‐latitude regions. This unexpected ionospheric diurnal variation has been studied for several decades. The underlying processes have been suggested to be related to meridional winds, topside influx arising from sunset ionospheric collapse, and other factors. However, substantial controversies remain unresolved. Using a numerical model driven by the statistical topside O+ diffusive flux from the Millstone Hill incoherent scatter radar data, we provide new insight into the competing roles of topside diffusive flux, neutral winds, and electric fields in forming the evening density peak. Simulations indicate that while meridional winds, which turn equatorward before sunset, are essential to sustain the daytime ionization near dusk, the topside diffusive flux is critically important for the formation and timing of the summer evening density peak. Plain Language Summary The phenomenon of mid‐latitude summer nighttime anomalous ionospheric electron density enhancement (MSNA) has often been observed by both ground‐based and space‐based measurements. Previous observation and simulation studies suggested that the combined effects of neutral winds and geomagnetic field configuration are the drivers of the formation of MSNA. These results, particularly some modeling results, were obtained under some constraints and assumptions that have not been strictly validated either through critical observations or self‐consistent, first principle simulations of the coupled ionosphere‐thermosphere system. This study represents a major new advance in understanding an important controlling factor that has not been adequately addressed before. In this study, we evaluate the relative roles of topside diffusive flux, meridional winds, and electric fields over Millstone Hill, where MSNA‐like anomalies are very representative of mid‐latitudes. Using a self‐consistent coupled ionosphere‐thermosphere model, Thermosphere–Ionosphere−Electrodynamics General Circulation Model, driven by the statistical topside O+ diffusive flux from the Millstone Hill Incoherent Scatter Radar data, we found that topside O+ diffusion is critical for the formation and timing of the evening electron density peak, although it is also very clear that meridional winds, which turn equatorward before sunset, are essential for sustaining the daytime ionization near dusk. This new finding from the Millstone Hill observations may lead to an ultimate understanding of the more generic MSNA features in both the Northern and Southern Hemispheres. Key Points The ionospheric electron density summer evening anomaly at Millstone Hill is investigated using a joint observation and simulation analysis Topside O+ diffusive flux is critical for anomaly formation and timing, while thermospheric winds act to preserve solar‐produced plasma A much better understanding of the mid‐latitude summer night anomaly is achieved with observation‐informed topside specification