Background: Lesbian, gay, bisexual, and asexual adolescents are at increased risk of poor mental health outcomes but it remains unknown whether this disparity emerges from early childhood and through to adolescence. This study tested sexual orientation disparities in the developmental trajectories of emotional and behavioral difficulties from ages 5 to 14 years, and the influence of early life adversities upon them. Methods: The Millennium Cohort Study, a British birth cohort, was used (4,838 boys and 5,016 girls). Parent-report emotional and behavioral difficulties at ages 5, 7, 11, and 14 were measured using the Strength and Difficulties Questionnaire. Sexual orientation was measured via sexual attraction at age 17. Early life adversities in the first 5 years of life, including parental age at birth, birthweight, duration of breastfeeding, parent-child relationship quality, and parental absence, were collected prospectively. Results: Latent growth modeling suggested that, for both sexes, sexual orientation disparities in emotional and behavioral difficulties increased from age 5 to 14 years. Homosexual and bisexual girls displayed significantly greater emotional and behavioral difficulties than heterosexual girls at ages 7, 11, and 14. Homosexual and bisexual boys displayed significantly greater emotional and behavioral difficulties than heterosexual boys at ages 11 and 14. Asexual boys and girls displayed significantly greater emotional and behavioral difficulties than their heterosexual counterparts at all four ages. For both sexes, sexual orientation disparities in emotional and behavioral difficulties only reduced slightly in magnitude after controlling for early life adversities. Conclusions: Sexual minority disparities in emotional and behavioral difficulties increased from childhood to early adolescence, possibly due to the accumulation of susceptibility to minority-related stressors. Sex and early life adversity may contribute to these developmental disparities.