Abstract Importance: There is potential for mental health status to act as a determinant of an individual’s ability to engage in healthful lifestyle behaviours. Objective: The objective of this study was to investigate the associations between parent-reported mental health problems during childhood, and self-reported mental health problems in adolescence, and health behaviours in adolescence. Design: Wave 4 and Wave 6 of the Millennium Cohort Study were utilised in this longitudinal cohort study which were collected in 2008 and 2015, respectively. Regression models were calculated for each lifestyle variable, with mental health change from 7 to 14 years as the exposure variable. Data were weighted to account for the potential clustering of region of sampling and adjusted for non-response. Setting: The Millennium Cohort Study is a UK population-representative longitudinal study of young people born during 2000-2001. Participants: This is a UK population-representative sample of young people (n=4,614). Half (52%) of the sample were female, most (77%) were born in England, and a greater proportion of the final sample had a household income reflective of the highest quintile (27%) compared to lowest quintile (8%). Most (66%) did not have mental health problems at baseline or follow-up. There were 24% who self-reported mental health problems at age 14 years, but did not have parent-reported mental health problems at 7 years. Exposure: Mental health problems at 7 years were parent-reported using Strengths and Difficulties Questionnaire. Mental health problems were self-reported at 14 years using the Short Mood and Feelings Questionnaire. Main outcome: Health behaviours at 14 years were the main outcome of interest. Sleep duration, fruit, vegetable, and soft drink consumption and social media use were self-reported using recall on a typical day. Results: The total participating sample was 4,618; 2,240 were female and 2,638 were recruited from England. Adolescents who self-reported mental health problems at 14 years were less likely to have at least 9 hours of sleep (OR=0.38, 95%CI:0.26;0.56), were less likely to consume fruit (OR=0.58, 95%CI:0.38;0.86) and reported greater use of social media (b=0.73, 95%CI:0.53;0.94), compared to individuals who did not have parent-reported mental health problems at 7 years nor self-reported at 14 years. Conclusions and Relevance: The presence of depressive symptoms at 14 years were associated to some health behaviours at 14 years. These findings are particularly important given that independent health behaviours can both deteriorate and become habitual during adolescence, and it is also a known time for the first emergence of mental health problems that continue into adulthood. Trial Registration: This study and analysis plan was registered on Australia New Zealand Clinical Trials Registry ACTRN12619001245189.