Mechanical complications (MCs) following acute myocardial infarction (AMI), such as ventricular septal rupture (VSR), free-wall rupture (FWR), and papillary muscle rupture (PMR), are fatal. However, the risk factors of in-hospital mortality among patients with MCs have not been previously reported in Japan. The purpose of this study was to evaluate the prognostic factors of in-hospital mortality in these patients. The study cohort consisted of 233 consecutive patients with MCs from the registry of 10 facilities in the Cardiovascular Research Consortium-8 Universities (CIRC-8U) in East Japan between 1997 and 2014 (2.3% of 10,278 AMI patients). The authors conducted a retrospective observational study to analyse the correlation between the subtypes of MCs with in-hospital mortality, clinical data, and medical treatment. We observed a decreasing incidence of MC (1997–2004: 3.7%, 2005–2010: 2.1%, 2011–2014: 1.9%, p < 0.001). In-hospital mortality among patients with MCs was 46%. Thirty-three percent of patients with MCs were not able to undergo surgical repair due to advanced age or severe cardiogenic shock. In-hospital mortality among patients who had undergone surgical repair was 29% (VSR: 21%, FWR: 33%, PMR: 60%). In patients with MCs, hazard ratio for in-hospital mortality according to multivariate analysis of without surgical repair was 5.63 (95% CI 3.54–8.95). In patients with surgical repair, the hazard ratios of blow-out-type FWR (5.53, 95% confidence interval (CI) 2.22–13.76), those with renal dysfunction (3.11, 95% CI 1.37–7.05), and those receiving venoarterial extracorporeal membrane oxygenation (VA-ECMO) (3.79, 95% CI 1.81–7.96) were significantly high. Although primary percutaneous coronary intervention (PCI) is associated with decreased incidence of MCs, high in-hospital mortality persisted in patients with MCs that also presented with renal dysfunction and in those requiring VA-ECMO. Early detection and surgical repair of MCs are essential.