BACKGROUND: There is increasing concern regarding the potential impact of social media use on the mental health of young people. Previous research has relied heavily on retrospective accounts of social media screen-time. Yet recent evidence suggests that such self-report measures are unreliable, correlating poorly with more objective measures of social media use. In principle, time use diaries provide a less biased measure of social media use. METHODS: We analysed cross-sectional data from the Millennium Cohort Study to explore associations between social media screen-time as recorded in time use diaries (TUD) and key mental health outcomes - self-harm in the past year, depressive symptoms (Short Mood and Feelings Questionnaire), self-esteem (shortened Rosenberg scale) - in adolescence. Social media TUD data were available for 4,032 participants (25.4% aged 13; 73.5% aged 14; 1.1% aged 15). RESULTS: Following adjustment for confounders, a greater amount of time spent on social media was associated with an increased risk of self-harm (adjusted OR per 30-minute increase in weekday use: 1.13, 95%CI 1.06 to 1.21) and depression (adjusted B ==0.36, 95%CI 0.22 to 0.50) and lower levels of self-esteem (adjusted B = -0.12, 95%CI -0.20 to -0.04) in females. Findings were similar for weekday and weekend use. LIMITATIONS: The cross-sectional nature of the data limits inference in relation to the causal direction of these associations. CONCLUSIONS: Future research should examine the direction of the associations with self-harm and other mental health outcomes and explore gender differences in how adolescents engage with social media as well as how much time they spend online.