Background Modern contraceptives are effective interventions for preventing unwanted pregnancies, but their use remain low among adolescent girls in sub-Saharan Africa. First, we aimed to assess the trends of modern contraceptive use among adolescent girls aged 15-19 years in Sierra Leone in the period 2008-2019. Second, we explored how individual and contextual determinants of their use changed during this period. Methods We used a combined sample of 8,774 adolescent girls aged 15-19 years from the 2008, 2013, and 2019 Sierra Leone Demographic and Health Surveys (SLDHS), respectively, to estimate the modern contraceptive prevalence rate (MCPR) and unmet need for family planning. We described the method mix and sources of contraceptives among modern method users. We then conducted a multivariable logistic regression to examine the determinants of modern contraceptive use among sexually active adolescent girls, separately for 2008 and 2019. Results MCPR increased from 5.9% (95% CI=4.5-7.8) in 2008 to 21.0% (95% CI=19.2-22.9) in 2019. Most of the increase had occurred by 2013. Norplant (48.8%) and injectables (39%) were the common methods in 2019. Unmet need for family planning declined from 15.4% (95% CI=13.2-17.9) in 2008 to 11.3% (95% CI=9.9-12.7) in 2019, while the prevalence of modern contraceptive use among sexually active adolescent girls increased from 13.1% (95% CI=10-17.0) in 2008 to 43.5% (95% CI=40.2-46.9) in 2019. In both the 2008 and 2019 surveys, the adjusted odds of using modern contraceptives among sexually active adolescent girls were higher among those with more than one sexual partner in the past 12 months, and with secondary or tertiary education. In 2019, the adjusted odds of using a modern contraceptive were also lower among adolescent girls who were married or living with a partner, had more than one child, and lived in the Western province. Conclusions The increase in contraceptive use among adolescent girls in Sierra Leone had stalled since 2013. Interventions, including increasing awareness and accessibility to modern contraceptives, are urgently needed, particularly for adolescents without formal education. Comprehensive and affordable community and school-based interventions, including adolescent-friendly services, would be helpful.