This study tested adolescent sexual behavior patterns at age 14, their association with mental health at age 17 (psychological well-being, substance use, and self-harm attempts), and the influence of early life adversities upon this association. A British birth cohort (5,593 boys and 5,724 girls from the Millennium Cohort Study) was used. Latent class analysis suggested five subgroups of adolescent sexual behaviors: a "no sexual behavior" (50.74%), a "kisser" (39.92%), a "touching under clothes" (4.71%), a "genital touching" (2.64%), and an "all sexual activities" class (1.99%). Adolescents from the "kisser," "touching under clothes," "genital touching," and "all sexual activities" classes reported significantly more substance use and self-harm attempts compared to adolescents from the "no sexual behavior" group. The associations became weaker after controlling for early life adversities (reducing around 4.38% to 37.35% for boys, and 9.29% to 52.56% for girls), and reduced to a smaller degree after further controlling for mental health variables at 14. The associations between sexual behaviors and psychological well-being became non-significant after controlling for early life adversities. Adolescents who have engaged in low-intensity sexual activities at early age may have poorer reported mental health, a pattern that is stronger for girls and early life adversity may partially explain this association.