Previous global and regional studies indicate that adolescents and young adults (i.e., youth) are affected by various mental disorders with lifelong consequences. However, there are no national estimates of mental disorders prevalent among Saudi youth. Using data from the Saudi National Mental Health Survey (SNMHS), we examined the lifetime prevalence, treatment, and socio-demographic correlates of mental disorders among Saudi youth (aged 15–30). A total of 4004 interviews were conducted using the adapted Composite International Diagnostic Interview (CIDI 3.0). Cross tabulations and logistic regression were used to generate estimates for the SNMHS youth sample (n = 1881). The prevalence of a mental disorder among Saudi youth was 40.10%, where anxiety disorders affected 26.84% of the sample, followed by disruptive behavior disorders (15.44%), mood disorders (9.67%), substance use disorders (4%) and eating disorders (7.06%). Sex, education, parental education, income, marital status, region, and family history of disorders were significant correlates of various classes of mental disorders. Only 14.47% of Saudi youth with any mental disorder received treatment for a lifetime disorder. Age, parental education, and family history of disorders emerged as significant correlates of mental health treatment. Lifetime mental disorders are highly prevalent among Saudi youth. There is an unmet need for culturally sensitive and age-appropriate treatment of lifetime mental disorders among youth in Saudi Arabia.