Objective: Symptoms of depression are highly prevalent and persistent following myocardial infarction (MI). Whether depression is a risk factor for long-term mortality following MI remains controversial. The present study aimed to determine whether depression during hospitalisation for acute MI (AMI) predicted 5-year all-cause or cardiac mortality. Method: This study utilised the Identifying Depression as a Comorbid Condition (IDACC) database of 337 hospitalised patients with AMI. Depressive symptoms were assessed using the Center for Epidemiologic Studies Depression scale (CES-D). Data were linked to a government administrative death registry to determine 5-year mortality. Survival data were analysed using Cox's proportional hazards model. Results: The mean age during AMI hospitalisation was 59 years ± 12, 74% of patients were men and depression (CES-D ≥ 16) was present in 132 patients (39.3%). The 5-year all-cause mortality rate was 10.4% (35 deaths) and the cardiac mortality rate was 6.5% (22 deaths). When depression was defined as a dichotomous variable, moderate to severe depression (defined by CES-D ≥ 27) at the time of AMI was associated with all-cause mortality (hazard ratio 2.54, 95% confidence interval 1.03 to 6.28; p = 0.04) but not cardiac mortality. However, when depression was defined by three categories (no depression CES-D < 16, mild depression CES-D 16-26, moderate to severe depression CES-D ≥ 27), it was not found to predict mortality. In addition, perceived social support was a predictor of all-cause and cardiac mortality in AMI patients. Conclusions: Our results indicate that the relationship between mortality and depression severity is not linear and that the association only becomes evident when the severity reaches a threshold level of CES-D ≥ 27, consistent with major depression. Low power may have influenced the finding of a lack of association between depression and cardiac mortality.