Using intergenerational, prospective data at ages 9 months, 7, 11, and 14 years from the nationally representative United Kingdom Millennium Cohort Study, this interdisciplinary study unpacks why 14‐year‐old adolescents with early perceived pubertal timing (PT) were more likely to drink alcohol (ever, frequent, and binge drinking) compared to those whose PT was on‐time or late (5,757 girls, 5,799 boys; 80% White, 10% Asian, 3% Black, and 7% Other British). Parents allowed drinking among 22% (18%) of early PT girls (boys) compared to 11% of late PT adolescents; formal mediation models showed differences by PT in parent permissiveness and gains in alcohol‐using friends primarily explained age 14 PT‐drinking associations. Parental alcohol permissiveness should be a key prevention target for early PT adolescents.