Purpose Aortic valve pathology in patients younger than 20 years is usually surgically managed with valve repair or the Ross procedure. Mechanical aortic valve replacement (AVR) becomes necessary if the aortic valve is irreparable, the repair fails, or the Ross procedure is contraindicated. This study evaluated outcomes after mechanical AVR in this group of patients. Methodology Retrospective cohort study of patients in Queensland who were younger than 20 years and underwent mechanical AVR between 1997 and 2020. Results From 1997 to 2020, 85 patients underwent mechanical AVR in Queensland [65 (76 %) male and 12 (14.1%) First Nations]. Median age was 13.5 years (IQR 10.5-15.4). 37 (44%) had prior aortic valve operation. Primary diagnoses were congenital defects (61%), rheumatic heart disease (RHD, 33%), and infective endocarditis (6%). Median size of the prosthesis was 22 mm (IQR 21-23). 17 (20%) required concomitant aortic root enlargement. There were two (2.3%) early deaths. Median follow-up was 8.71 years (IQR 4.5-14.4). 24 (14%) needed re-replacement of the valve prosthesis. 12 (14%) patients died during follow-up. Kaplan-Meier survival was 91.6%, 89.4%, and 76.3%, and freedom from aortic valve reintervention was 96.2%, 89.4%, and 87.5% at 5, 10 and 20 years, respectively. Warfarin compliance was good in 77 (90%) and poor in 8 (10%) patients. Nine (11%) patients had a thromboembolic event (stroke 6; transient ischemic attack 2; small bowel ischemia 1; 8 were poorly compliant with warfarin). 68 (n=73, 93.1%) non-First Nations patients and 9 (n=12, 75%) First Nations patients were compliant with warfarin (p=0.04). Conclusion Mechanical aortic valve replacement in patients younger than 20 has a low early mortality. 1/6th require re-replacement, and 14% do not survive to adulthood. Warfarin compliance is poor in 10% of patients in Queensland. It is worse in First Nations patients and is associated with thromboembolic complications.