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Poor sleep and mental health are closely linked in adolescence, yet most studies rely on self-reported sleep. We investigated cross-sectional associations between objective sleep features (sleep duration, duration variability, and onset time) and mental health outcomes in 1371 U.S. adolescents from the Adolescent Brain Cognitive Development℠ Study (ABCD^®) [year 4 follow-up; mean age 14.09 ± 0.69, 51.86% female], controlling for age, sex, race/ethnicity, and parental education. Mental health outcomes were internalizing symptoms (emotional distress directed inward) and emotion regulation, including cognitive reappraisal (reframing emotional situations) and expressive suppression (inhibiting outward expression). Sleeping ≤7.05 h (β = 0.06 [95% CI: 0.00-0.11], p = 0.036), higher sleep duration variability (β = 0.09 [0.03-0.14], p = 0.001), with stronger effects in females (interaction: β = -0.12 [-0.22 to -0.02], p = 0.020), and later sleep onset times (β = 0.06 [0.01-0.11], p = 0.030) were all associated with greater internalizing symptoms. Sleep was not linked to reappraisal (all p > 0.05). Higher variability was associated with greater suppression (β = 0.09 [0.03-0.14], p = 0.001). Conclusively, objective sleep features were differentially associated with mental health outcomes.