Background and Aims: Recent research suggests that parental supply of alcohol is associated with more risky drinking and alcohol-related harm among adolescents. However, the overall effect of parental supply throughout adolescence remains unclear, because parental supply of alcohol varies during adolescence. Due to the complexity of longitudinal data, standard analytical methods can be biased. This study examined the effect of parental supply of alcohol on alcohol-related outcomes in early adulthood using robust methods to minimize risk of bias. Design: Prospective longitudinal cohort study. Setting: Australia. Participants: A cohort of school students (n = 1906) recruited in the first year of secondary school (average age 12.9 years) from Australian schools in 2010–11, interviewed annually for 7 years. Measurements: The exposure variable was self-reported parental supply of alcohol (including sips/whole drinks) during 5 years of adolescence (waves 1–5). Outcome variables were self-reported binge drinking, alcohol-related harm and symptoms of alcohol use disorder, measured in the two waves after the exposure period (waves 6–7). To reduce risk of bias, we used targeted maximum likelihood estimation to assess the (counterfactual) effect of parental supply of alcohol in all five waves versus no supply on alcohol-related outcomes. Findings: Parental supply of alcohol throughout adolescence saw greater risk of binge drinking [risk ratios (RR) = 1.53; 95% confidence interval (CI) = 1.27–1.84] and alcohol-related harms (RR = 1.44; 95% CI = 1.22–1.69) in the year following the exposure period compared with no supply in adolescence. Earlier initiation of parental supply also increased risk of binge drinking (RR = 1.10; 95% CI = 1.05–1.14), and any alcohol-related harm (RR = 1.09; 95% CI = 1.05–1.13) for each year earlier parental supply began compared with later (or no) initiation. Conclusions: Adolescents whose parents supply them with alcohol appear to have an increased risk of alcohol-related harm compared with adolescents whose parents do not supply them with alcohol. The risk appears to increase with earlier initiation of supply. © 2020 Society for the Study of Addiction