Abstract Aims As there is limited evidence regarding the prognostic impact of prior left ventricular ejection fraction (LVEF) in patients with heart failure with mildly reduced ejection fraction (HFmrEF), this study investigates the prognostic impact of longitudinal changes in LVEF in patients with HFmrEF. Methods Consecutive patients with HFmrEF (i.e. LVEF 41–49% with signs and/or symptoms of HF) were included retrospectively in a monocentric registry from 2016 to 2022. Based on prior LVEF, patients were categorized into three groups: stable LVEF, improved LVEF, and deteriorated LVEF. The primary endpoint was 30-months all-cause mortality (median follow-up). Secondary endpoints included in-hospital and 12-months all-cause mortality, as well as HF-related rehospitalization at 12 and 30 months. Kaplan–Meier and multivariable Cox proportional regression analyses were applied for statistics. Results Six hundred eighty-nine patients with HFmrEF were included. Compared to their prior LVEF, 24%, 12%, and 64% had stable, improved, and deteriorated LVEF, respectively. None of the three LVEF groups was associated with all-cause mortality at 12 ( p ≥ 0.583) and 30 months (31% vs. 37% vs. 34%; log rank p ≥ 0.376). In addition, similar rates of 12- ( p ≥ 0.533) and 30-months HF-related rehospitalization (21% vs. 23% vs. 21%; log rank p ≥ 0.749) were observed. These findings were confirmed in multivariable regression analyses in the entire study cohort. Conclusion The transition from HFrEF and HFpEF towards HFmrEF is very common. However, prior LVEF was not associated with prognosis, likely due to the persistently high dynamic nature of LVEF in the follow-up period. Graphical Abstract