Background and Objective: Primary aldosteronism (PA), characterised by excessive aldosterone production and mineralocorticoid receptor activation, is associated with adverse cardiovascular outcomes including left ventricular hypertrophy and failure. Elevated cardiovascular risk is observed even in milder and subclinical forms of PA, based on studies primarily conducted in middle-aged and elderly populations. It is unclear if the biomarkers of PA, including renin, aldosterone, and aldosterone-to-renin ratio (ARR), are associated with left ventricular mass index (LVMI), which is a strong predictor of cardiovascular diseases, in young adults. Methods: The Raine Study is a longitudinal, population-based cohort study in Western Australia that enrolled women during pregnancy. We analysed the data from the offspring of these women at age 27 years. Participants with elevated high-sensitivity C-reactive protein (>10 mg/L) and females who were on oral contraception were excluded. LVMI was measured by cardiac magnetic resonance imaging. We examined the relationship between LVMI and plasma renin, aldosterone, and ARR using multivariate-adjusted linear regression and used mediation analysis to test whether these relationships were dependent on systolic blood pressure (SBP). Results: Of 758 participants, 252 (33.2%) were females. Females had lower median plasma renin concentration (12.0 vs 15.4mU/L; p<0.001) and higher median ARR (21.0 vs 15.6; p<0.001) than males. There was no sex-difference in median plasma aldosterone concentration (257 vs 237pmol/L in males; p=0.143). Females had lower LVMI (52.2 vs 70.2 g/m2; p<0.001) than males. A significant association between LVMI and aldosterone was detected in males (β-coefficient 0.009; 95% CI 0.001 to 0.017; p=0.027), and between LVMI and ARR in females (β-coefficient 0.098; 95% CI 0.001 to 0.196; p=0.050) after adjusting for confounders. This relationship was not mediated by brachial SBP. Conclusions: An elevated aldosterone concentration or ARR was associated with higher LVMI in young adults, independent of brachial SBP. Long-term follow-up is required to determine if the relationship persists over time and leads to reduced cardiac function.