BACKGROUND: Mental and substance use disorders are a major public health concern globally, with high rates of disability, morbidity, and mortality associated with these. In low- and middle-income countries, such as Kenya, mental health is often given low priority, and resources for the prevention and treatment of mental and substance use disorders are limited. Adolescence and young adulthood are critical periods for the development of mental and substance use disorders, with many disorders emerging during this time. In Kenya, the burden and risk factors of mental and substance use disorders among adolescents and young adults is not well understood. METHODS: The data used in this study were obtained from the Global Burden of Disease (GBD) Study 2019. We selected the data on the number of mental and substance use disorders among adolescents and young adults in Kenya from the GBD results tool. The data were extracted by mental health (MH) condition, by age group and by sex. We used descriptive statistical methods to summarise and present the data. Specifically, we calculated the disability-adjusted life-years (DALYs) rates, risk factors of mental and substance use disorders by age group and sex. FINDINGS: In 2019, among 10-24-year-olds in Kenya, mental disorders ranked as the second leading cause of disability, following unintentional injuries, and accounted for 248,936 [95% uncertainty interval 175,033; 341,680] DALYs or 9.4% of 2,656,546 total DALYs. Substance use disorders accounted 15,022 [9948; 20,710] DALYs. Depressive, anxiety, and conduct disorders accounted for the most DALYs of mental disorders accounting for 3.1%, 2.3% and 1.7% of the total DALYs, respectively. The main risk factors for incident DALYs in 10-24-year-olds were bullying and victimization (66.5%). Childhood sexual abuse accounted for 13.7% of the DALYs, lead exposure accounted for 8.5% of the DALYs, intimate partner violence accounted for 11.3% of the DALYs (2%) with all victims being females, and illicit drug use accounted for (52.7%) of DALYs. INTERPRETATION: Improved surveillance of mental health and substance use burden at national and county levels is needed. Focus on timely screening and intervention for idiopathic developmental intellectual disability, conduct disorder, and substance use disorder in young boys and depression, anxiety, and eating disorders in young girls and women is critically needed. FUNDING: MK is funded by FIC/NIMH K43 TW 010716 and R33MH124149-03. The publication was made possible by funding from the Gates Foundation.