Background: Mental illnesses following an acute myocardial infarction (AMI) are a growing concern, as they are associated with worse outcomes for AMI patients. Our understanding of the prevalence of mental illnesses after an AMI is incomplete, as most studies investigate depression while overlooking other conditions like anxiety and PTSD. Existing studies often rely on patient-reported questionnaires for mental illness diagnoses, a method that can be subjective. To address this, we conducted a systematic review and meta-analysis to determine the prevalence and risk factors of depression, anxiety, and PTSD after AMI, including only studies with formal mental illness diagnoses. Methods: Searches in MEDLINE, EMBASE, and PsycINFO up to 23 January 2025 identified 23 qualifying studies that assessed the prevalence of depression, anxiety, and PTSD after AMI, with cases defined exclusively by formal diagnoses established through psychiatrist-administered structured interviews according to the Diagnostic and Statistical Manual for Mental Disorders (DSM) criteria (versions III to V). For each outcome, the pooled prevalence was estimated using meta-analyses of proportions with random-effects models. If significant heterogeneity was detected, subgroup analyses and meta-regression were performed to explore the factors contributing to this heterogeneity. Results: A total of 25 studies were included in the meta-analysis. Among the 20 studies included, the pooled prevalence of depression after AMI was 23.58% (95% CI: 22.86%; 24.32%). When stratified by time since AMI, the prevalence was 19.46% (95% CI: 15.47%; 24.19%) for those assessed within 3 months and 14.87% (95% CI: 9.55%; 22.43%) for those assessed after 3 months. The pooled prevalence of anxiety (seven studies) and PTSD (three studies) was 11.96% (95% CI: 6.15; 21.96%) and 10.26% (95% CI: 5.49%; 18.36%), respectively. Further pooled prevalence subgroup analysis of depression and anxiety revealed significantly higher rates in the female sex (29.89%, 95% CI: 21.85; 39.41%), in those with hypertension (25.01%, 95% CI: 21.68; 28.67%), diabetes (25.01%, 95% CI: 21.68; 28.67%), or hyperlipidemia (28.96% 95% CI: 23.44; 35.17%), and in smokers (25.23%., 95% CI: 19.48; 32.00%), while the pooled prevalence of depression is higher in unmarried (35.44%, 95% CI: 19.61; 55.26%) than married individuals (28.63%, 95% CI: 18.67; 41.20%) and in those with a history of depression (57.41%, 95% CI: 31.47; 78.92%). The results of the meta-regression indicated that a prior history of depression was a significant predictor of depression prevalence (p = 0.0035, regression coefficient 1.54). Conclusions: The prevalence of mental illnesses, including depression, anxiety, and PTSD, is notable following an AMI. Identified risk factors encompass female sex, hypertension, diabetes mellitus, hyperlipidemia, smoking, a history of depressive illness, and social context.