Background: Approximately 5%–10% of acute coronary syndrome (ACS) cases occur in people younger than 45 years of age. This study aimed to identify complications of ACS and the associated factors in young patients. Methods: In this cross-sectional study, data from 147 ACS patients aged less than 45 years were analysed. Results: The mean age was 39.1 (4.9) years, the male to female ratio was 3:1; 21.2% of patients presented with unstable angina, 58.5% had non-ST elevation myocardial infarction and 20.4% had ST elevation myocardial infarction. The most frequent risk factor of ACS was dyslipidaemia (65.3%), followed by hypertension (43.5%). In total, 49.7% of patients had inpatient complication(s), with the most common being heart failure (35.4%), followed by arrhythmia (20.4%). The significant factors associated with ACS complications were current smoking [adjusted odds ratio (AOR) 4.03; 95% confidence interval (CI): 1.33, 12.23; P-value = 0.014], diabetic mellitus [AOR 3.03; 95% CI: 1.19, 7.71; P-value = 0.020], treatments of fondaparinux [AOR 0.18; 95% CI: 0.08, 0.39; P-value < 0.001] and oral nitrates [AOR 0.18; 95% CI: 0.08, 0.42; P-value < 0.001]. Conclusions: Smoking status and diabetes mellitus were modifiable risk factors while pharmacological treatment was an important protective factor for ACS complications in young patients.