Background: Recent data suggests that the majority of cardiac deaths in patients with heart failure occur in patients with a left ventricular ejection fraction (LVEF) >35%. This study sought to determine the value of guideline based assessment of diastolic dysfunction in predicting all-cause mortality in patients with a first-ever myocardial infarction (MI) with an LVEF >35%. Methods: A retrospective single centre study involving 383 patients with a first-ever MI (STEMI or NSTEMI) with LVEF >35% was performed. Clinical, angiographic and echocardiographic data were obtained from prospectively maintained institutional databases. Outcomes data were obtained from national death registry. Echocardiography was performed early post-admission for all patients. Significant diastolic dysfunction (DD) was defined was grade 2/3 diastolic dysfunction according to current American Society of Echocardiography/European Association of Cardiovascular Imaging guidelines. Results: At a median follow up of 2 years, there were 32 deaths. On Cox proportional hazards multivariate analysis incorporating significant clinical variables (age, chronic kidney disease and extent of coronary artery disease), significant DD (HR 2.57, 95%CI 1.16–5.68, p = 0.020) and left ventricular end-diastolic volume index (HR 1.03, 1.04–1.07, p = 0.021) were the only independent echocardiographic predictors of all-cause mortality. Intermodel comparisons using model χ2 and Harrel's-C confirmed incremental value of DD. In the subgroup with LVEF 36–55% (n = 176), significant DD was the only independent echocardiographic predictor (HR 3.56, 95%CI 2.46–9.09, p = 0.006). Conclusions: The presence of significant DD identifies patients with LVEF >35% following MI who are at a higher risk of all-cause mortality, and who may benefit from further risk stratification and treatment.